diff --git a/.eslintrc.js b/.eslintrc.js
index 0732923e..fc169abe 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -41,7 +41,7 @@ module.exports = {
     },
     // Packages targeting DOM
     {
-      files: ['packages/{vue,runtime-dom}/**'],
+      files: ['packages/{vue,vue-compat,runtime-dom}/**'],
       rules: {
         'no-restricted-globals': ['error', ...NodeGlobals]
       }
diff --git a/jest.config.js b/jest.config.js
index e8507b5d..306ed5f0 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -12,7 +12,8 @@ module.exports = {
     __NODE_JS__: true,
     __FEATURE_OPTIONS_API__: true,
     __FEATURE_SUSPENSE__: true,
-    __FEATURE_PROD_DEVTOOLS__: false
+    __FEATURE_PROD_DEVTOOLS__: false,
+    __COMPAT__: true
   },
   coverageDirectory: 'coverage',
   coverageReporters: ['html', 'lcov', 'text'],
@@ -34,6 +35,7 @@ module.exports = {
   watchPathIgnorePatterns: ['/node_modules/', '/dist/', '/.git/'],
   moduleFileExtensions: ['ts', 'tsx', 'js', 'json'],
   moduleNameMapper: {
+    '@vue/compat': '<rootDir>/packages/vue-compat/src',
     '^@vue/(.*?)$': '<rootDir>/packages/$1/src',
     vue: '<rootDir>/packages/vue/src'
   },
diff --git a/packages/compiler-core/__tests__/parse.spec.ts b/packages/compiler-core/__tests__/parse.spec.ts
index 790123ac..1c7c8785 100644
--- a/packages/compiler-core/__tests__/parse.spec.ts
+++ b/packages/compiler-core/__tests__/parse.spec.ts
@@ -1736,20 +1736,26 @@ foo
     })
   })
 
-  describe('whitespace management', () => {
+  describe('whitespace management when adopting strategy condense', () => {
+    const parse = (content: string, options?: ParserOptions) =>
+      baseParse(content, {
+        whitespace: 'condense',
+        ...options
+      })
+
     it('should remove whitespaces at start/end inside an element', () => {
-      const ast = baseParse(`<div>   <span/>    </div>`)
+      const ast = parse(`<div>   <span/>    </div>`)
       expect((ast.children[0] as ElementNode).children.length).toBe(1)
     })
 
     it('should remove whitespaces w/ newline between elements', () => {
-      const ast = baseParse(`<div/> \n <div/> \n <div/>`)
+      const ast = parse(`<div/> \n <div/> \n <div/>`)
       expect(ast.children.length).toBe(3)
       expect(ast.children.every(c => c.type === NodeTypes.ELEMENT)).toBe(true)
     })
 
     it('should remove whitespaces adjacent to comments', () => {
-      const ast = baseParse(`<div/> \n <!--foo--> <div/>`)
+      const ast = parse(`<div/> \n <!--foo--> <div/>`)
       expect(ast.children.length).toBe(3)
       expect(ast.children[0].type).toBe(NodeTypes.ELEMENT)
       expect(ast.children[1].type).toBe(NodeTypes.COMMENT)
@@ -1757,7 +1763,7 @@ foo
     })
 
     it('should remove whitespaces w/ newline between comments and elements', () => {
-      const ast = baseParse(`<div/> \n <!--foo--> \n <div/>`)
+      const ast = parse(`<div/> \n <!--foo--> \n <div/>`)
       expect(ast.children.length).toBe(3)
       expect(ast.children[0].type).toBe(NodeTypes.ELEMENT)
       expect(ast.children[1].type).toBe(NodeTypes.COMMENT)
@@ -1765,7 +1771,7 @@ foo
     })
 
     it('should NOT remove whitespaces w/ newline between interpolations', () => {
-      const ast = baseParse(`{{ foo }} \n {{ bar }}`)
+      const ast = parse(`{{ foo }} \n {{ bar }}`)
       expect(ast.children.length).toBe(3)
       expect(ast.children[0].type).toBe(NodeTypes.INTERPOLATION)
       expect(ast.children[1]).toMatchObject({
@@ -1776,7 +1782,7 @@ foo
     })
 
     it('should NOT remove whitespaces w/o newline between elements', () => {
-      const ast = baseParse(`<div/> <div/> <div/>`)
+      const ast = parse(`<div/> <div/> <div/>`)
       expect(ast.children.length).toBe(5)
       expect(ast.children.map(c => c.type)).toMatchObject([
         NodeTypes.ELEMENT,
@@ -1788,7 +1794,7 @@ foo
     })
 
     it('should condense consecutive whitespaces in text', () => {
-      const ast = baseParse(`   foo  \n    bar     baz     `)
+      const ast = parse(`   foo  \n    bar     baz     `)
       expect((ast.children[0] as TextNode).content).toBe(` foo bar baz `)
     })
 
@@ -1824,6 +1830,84 @@ foo
     })
   })
 
+  describe('whitespace management when adopting strategy preserve', () => {
+    const parse = (content: string, options?: ParserOptions) =>
+      baseParse(content, {
+        whitespace: 'preserve',
+        ...options
+      })
+
+    it('should still remove whitespaces at start/end inside an element', () => {
+      const ast = parse(`<div>   <span/>    </div>`)
+      expect((ast.children[0] as ElementNode).children.length).toBe(1)
+    })
+
+    it('should preserve whitespaces w/ newline between elements', () => {
+      const ast = parse(`<div/> \n <div/> \n <div/>`)
+      expect(ast.children.length).toBe(5)
+      expect(ast.children.map(c => c.type)).toMatchObject([
+        NodeTypes.ELEMENT,
+        NodeTypes.TEXT,
+        NodeTypes.ELEMENT,
+        NodeTypes.TEXT,
+        NodeTypes.ELEMENT
+      ])
+    })
+
+    it('should preserve whitespaces adjacent to comments', () => {
+      const ast = parse(`<div/> \n <!--foo--> <div/>`)
+      expect(ast.children.length).toBe(5)
+      expect(ast.children.map(c => c.type)).toMatchObject([
+        NodeTypes.ELEMENT,
+        NodeTypes.TEXT,
+        NodeTypes.COMMENT,
+        NodeTypes.TEXT,
+        NodeTypes.ELEMENT
+      ])
+    })
+
+    it('should preserve whitespaces w/ newline between comments and elements', () => {
+      const ast = parse(`<div/> \n <!--foo--> \n <div/>`)
+      expect(ast.children.length).toBe(5)
+      expect(ast.children.map(c => c.type)).toMatchObject([
+        NodeTypes.ELEMENT,
+        NodeTypes.TEXT,
+        NodeTypes.COMMENT,
+        NodeTypes.TEXT,
+        NodeTypes.ELEMENT
+      ])
+    })
+
+    it('should preserve whitespaces w/ newline between interpolations', () => {
+      const ast = parse(`{{ foo }} \n {{ bar }}`)
+      expect(ast.children.length).toBe(3)
+      expect(ast.children[0].type).toBe(NodeTypes.INTERPOLATION)
+      expect(ast.children[1]).toMatchObject({
+        type: NodeTypes.TEXT,
+        content: ' '
+      })
+      expect(ast.children[2].type).toBe(NodeTypes.INTERPOLATION)
+    })
+
+    it('should preserve whitespaces w/o newline between elements', () => {
+      const ast = parse(`<div/> <div/> <div/>`)
+      expect(ast.children.length).toBe(5)
+      expect(ast.children.map(c => c.type)).toMatchObject([
+        NodeTypes.ELEMENT,
+        NodeTypes.TEXT,
+        NodeTypes.ELEMENT,
+        NodeTypes.TEXT,
+        NodeTypes.ELEMENT
+      ])
+    })
+
+    it('should preserve consecutive whitespaces in text', () => {
+      const content = `   foo  \n    bar     baz     `
+      const ast = parse(content)
+      expect((ast.children[0] as TextNode).content).toBe(content)
+    })
+  })
+
   describe('Errors', () => {
     const patterns: {
       [key: string]: Array<{
diff --git a/packages/compiler-core/__tests__/transforms/vIf.spec.ts b/packages/compiler-core/__tests__/transforms/vIf.spec.ts
index f1b91cc4..2024cdd3 100644
--- a/packages/compiler-core/__tests__/transforms/vIf.spec.ts
+++ b/packages/compiler-core/__tests__/transforms/vIf.spec.ts
@@ -306,6 +306,7 @@ describe('compiler: v-if', () => {
           code: ErrorCodes.X_V_IF_SAME_KEY
         }
       ])
+      expect('unnecessary key usage on v-if').toHaveBeenWarned()
     })
   })
 
diff --git a/packages/compiler-core/package.json b/packages/compiler-core/package.json
index 8adc7a6d..0297bc14 100644
--- a/packages/compiler-core/package.json
+++ b/packages/compiler-core/package.json
@@ -11,6 +11,7 @@
   ],
   "buildOptions": {
     "name": "VueCompilerCore",
+    "compat": true,
     "formats": [
       "esm-bundler",
       "cjs"
diff --git a/packages/compiler-core/src/ast.ts b/packages/compiler-core/src/ast.ts
index 4a2b610f..901f00ce 100644
--- a/packages/compiler-core/src/ast.ts
+++ b/packages/compiler-core/src/ast.ts
@@ -109,6 +109,9 @@ export interface RootNode extends Node {
   temps: number
   ssrHelpers?: symbol[]
   codegenNode?: TemplateChildNode | JSChildNode | BlockStatement
+
+  // v2 compat only
+  filters?: string[]
 }
 
 export type ElementNode =
diff --git a/packages/compiler-core/src/codegen.ts b/packages/compiler-core/src/codegen.ts
index e3dbbc82..d2322f41 100644
--- a/packages/compiler-core/src/codegen.ts
+++ b/packages/compiler-core/src/codegen.ts
@@ -50,7 +50,8 @@ import {
   CREATE_BLOCK,
   OPEN_BLOCK,
   CREATE_STATIC,
-  WITH_CTX
+  WITH_CTX,
+  RESOLVE_FILTER
 } from './runtimeHelpers'
 import { ImportItem } from './transform'
 
@@ -274,6 +275,12 @@ export function generate(
       newline()
     }
   }
+  if (__COMPAT__ && ast.filters && ast.filters.length) {
+    newline()
+    genAssets(ast.filters, 'filter', context)
+    newline()
+  }
+
   if (ast.temps > 0) {
     push(`let `)
     for (let i = 0; i < ast.temps; i++) {
@@ -458,11 +465,15 @@ function genModulePreamble(
 
 function genAssets(
   assets: string[],
-  type: 'component' | 'directive',
+  type: 'component' | 'directive' | 'filter',
   { helper, push, newline }: CodegenContext
 ) {
   const resolver = helper(
-    type === 'component' ? RESOLVE_COMPONENT : RESOLVE_DIRECTIVE
+    __COMPAT__ && type === 'filter'
+      ? RESOLVE_FILTER
+      : type === 'component'
+        ? RESOLVE_COMPONENT
+        : RESOLVE_DIRECTIVE
   )
   for (let i = 0; i < assets.length; i++) {
     let id = assets[i]
@@ -727,13 +738,11 @@ function genExpressionAsPropertyKey(
 }
 
 function genComment(node: CommentNode, context: CodegenContext) {
-  if (__DEV__) {
-    const { push, helper, pure } = context
-    if (pure) {
-      push(PURE_ANNOTATION)
-    }
-    push(`${helper(CREATE_COMMENT)}(${JSON.stringify(node.content)})`, node)
+  const { push, helper, pure } = context
+  if (pure) {
+    push(PURE_ANNOTATION)
   }
+  push(`${helper(CREATE_COMMENT)}(${JSON.stringify(node.content)})`, node)
 }
 
 function genVNodeCall(node: VNodeCall, context: CodegenContext) {
diff --git a/packages/compiler-core/src/compat/compatConfig.ts b/packages/compiler-core/src/compat/compatConfig.ts
new file mode 100644
index 00000000..d43c441d
--- /dev/null
+++ b/packages/compiler-core/src/compat/compatConfig.ts
@@ -0,0 +1,167 @@
+import { SourceLocation } from '../ast'
+import { CompilerError } from '../errors'
+import { ParserContext } from '../parse'
+import { TransformContext } from '../transform'
+
+export type CompilerCompatConfig = Partial<
+  Record<CompilerDeprecationTypes, boolean | 'suppress-warning'>
+> & {
+  MODE?: 2 | 3
+}
+
+export interface CompilerCompatOptions {
+  compatConfig?: CompilerCompatConfig
+}
+
+export const enum CompilerDeprecationTypes {
+  COMPILER_IS_ON_ELEMENT = 'COMPILER_IS_ON_ELEMENT',
+  COMPILER_V_BIND_SYNC = 'COMPILER_V_BIND_SYNC',
+  COMPILER_V_BIND_PROP = 'COMPILER_V_BIND_PROP',
+  COMPILER_V_BIND_OBJECT_ORDER = 'COMPILER_V_BIND_OBJECT_ORDER',
+  COMPILER_V_ON_NATIVE = 'COMPILER_V_ON_NATIVE',
+  COMPILER_V_IF_V_FOR_PRECEDENCE = 'COMPILER_V_IF_V_FOR_PRECEDENCE',
+  COMPILER_V_FOR_REF = 'COMPILER_V_FOR_REF',
+  COMPILER_NATIVE_TEMPLATE = 'COMPILER_NATIVE_TEMPLATE',
+  COMPILER_INLINE_TEMPLATE = 'COMPILER_INLINE_TEMPLATE',
+  COMPILER_FILTERS = 'COMPILER_FILTER'
+}
+
+type DeprecationData = {
+  message: string | ((...args: any[]) => string)
+  link?: string
+}
+
+const deprecationData: Record<CompilerDeprecationTypes, DeprecationData> = {
+  [CompilerDeprecationTypes.COMPILER_IS_ON_ELEMENT]: {
+    message:
+      `Platform-native elements with "is" prop will no longer be ` +
+      `treated as components in Vue 3 unless the "is" value is explicitly ` +
+      `prefixed with "vue:".`,
+    link: `https://v3.vuejs.org/guide/migration/custom-elements-interop.html`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_V_BIND_SYNC]: {
+    message: key =>
+      `.sync modifier for v-bind has been removed. Use v-model with ` +
+      `argument instead. \`v-bind:${key}.sync\` should be changed to ` +
+      `\`v-model:${key}\`.`,
+    link: `https://v3.vuejs.org/guide/migration/v-model.html`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_V_BIND_PROP]: {
+    message:
+      `.prop modifier for v-bind has been removed and no longer necessary. ` +
+      `Vue 3 will automatically set a binding as DOM property when appropriate.`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_V_BIND_OBJECT_ORDER]: {
+    message:
+      `v-bind="obj" usage is now order sensitive and behaves like JavaScript ` +
+      `object spread: it will now overwrite an existing non-mergeable attribute ` +
+      `that appears before v-bind in the case of conflict. ` +
+      `To retain 2.x behavior, move v-bind to make it the first attribute. ` +
+      `You can also suppress this warning if the usage is intended.`,
+    link: `https://v3.vuejs.org/guide/migration/v-bind.html`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_V_ON_NATIVE]: {
+    message: `.native modifier for v-on has been removed as is no longer necessary.`,
+    link: `https://v3.vuejs.org/guide/migration/v-on-native-modifier-removed.html`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_V_IF_V_FOR_PRECEDENCE]: {
+    message:
+      `v-if / v-for precedence when used on the same element has changed ` +
+      `in Vue 3: v-if now takes higher precedence and will no longer have ` +
+      `access to v-for scope variables. It is best to avoid the ambiguity ` +
+      `with <template> tags or use a computed property that filters v-for ` +
+      `data source.`,
+    link: `https://v3.vuejs.org/guide/migration/v-if-v-for.html`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_V_FOR_REF]: {
+    message:
+      `Ref usage on v-for no longer creates array ref values in Vue 3. ` +
+      `Consider using function refs or refactor to avoid ref usage altogether.`,
+    link: `https://v3.vuejs.org/guide/migration/array-refs.html`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_NATIVE_TEMPLATE]: {
+    message:
+      `<template> with no special directives will render as a native template ` +
+      `element instead of its inner content in Vue 3.`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_INLINE_TEMPLATE]: {
+    message: `"inline-template" has been removed in Vue 3.`,
+    link: `https://v3.vuejs.org/guide/migration/inline-template-attribute.html`
+  },
+
+  [CompilerDeprecationTypes.COMPILER_FILTERS]: {
+    message:
+      `filters have been removed in Vue 3. ` +
+      `The "|" symbol will be treated as native JavaScript bitwise OR operator. ` +
+      `Use method calls or computed properties instead.`,
+    link: `https://v3.vuejs.org/guide/migration/filters.html`
+  }
+}
+
+function getCompatValue(
+  key: CompilerDeprecationTypes | 'MODE',
+  context: ParserContext | TransformContext
+) {
+  const config = (context as ParserContext).options
+    ? (context as ParserContext).options.compatConfig
+    : (context as TransformContext).compatConfig
+  const value = config && config[key]
+  if (key === 'MODE') {
+    return value || 3 // compiler defaults to v3 behavior
+  } else {
+    return value
+  }
+}
+
+export function isCompatEnabled(
+  key: CompilerDeprecationTypes,
+  context: ParserContext | TransformContext
+) {
+  const mode = getCompatValue('MODE', context)
+  const value = getCompatValue(key, context)
+  // in v3 mode, only enable if explicitly set to true
+  // otherwise enable for any non-false value
+  return mode === 3 ? value === true : value !== false
+}
+
+export function checkCompatEnabled(
+  key: CompilerDeprecationTypes,
+  context: ParserContext | TransformContext,
+  loc: SourceLocation | null,
+  ...args: any[]
+): boolean {
+  const enabled = isCompatEnabled(key, context)
+  if (__DEV__ && enabled) {
+    warnDeprecation(key, context, loc, ...args)
+  }
+  return enabled
+}
+
+export function warnDeprecation(
+  key: CompilerDeprecationTypes,
+  context: ParserContext | TransformContext,
+  loc: SourceLocation | null,
+  ...args: any[]
+) {
+  const val = getCompatValue(key, context)
+  if (val === 'suppress-warning') {
+    return
+  }
+  const { message, link } = deprecationData[key]
+  const msg = `(deprecation ${key}) ${
+    typeof message === 'function' ? message(...args) : message
+  }${link ? `\n  Details: ${link}` : ``}`
+
+  const err = new SyntaxError(msg) as CompilerError
+  err.code = key
+  if (loc) err.loc = loc
+  context.onWarn(err)
+}
diff --git a/packages/compiler-core/src/compat/transformFilter.ts b/packages/compiler-core/src/compat/transformFilter.ts
new file mode 100644
index 00000000..bd6fd384
--- /dev/null
+++ b/packages/compiler-core/src/compat/transformFilter.ts
@@ -0,0 +1,194 @@
+import { RESOLVE_FILTER } from '../runtimeHelpers'
+import {
+  AttributeNode,
+  DirectiveNode,
+  NodeTransform,
+  NodeTypes,
+  SimpleExpressionNode,
+  toValidAssetId,
+  TransformContext
+} from '@vue/compiler-core'
+import {
+  CompilerDeprecationTypes,
+  isCompatEnabled,
+  warnDeprecation
+} from './compatConfig'
+import { ExpressionNode } from '../ast'
+
+const validDivisionCharRE = /[\w).+\-_$\]]/
+
+export const transformFilter: NodeTransform = (node, context) => {
+  if (!isCompatEnabled(CompilerDeprecationTypes.COMPILER_FILTERS, context)) {
+    return
+  }
+
+  if (node.type === NodeTypes.INTERPOLATION) {
+    // filter rewrite is applied before expression transform so only
+    // simple expressions are possible at this stage
+    rewriteFilter(node.content, context)
+  }
+
+  if (node.type === NodeTypes.ELEMENT) {
+    node.props.forEach((prop: AttributeNode | DirectiveNode) => {
+      if (
+        prop.type === NodeTypes.DIRECTIVE &&
+        prop.name !== 'for' &&
+        prop.exp
+      ) {
+        rewriteFilter(prop.exp, context)
+      }
+    })
+  }
+}
+
+function rewriteFilter(node: ExpressionNode, context: TransformContext) {
+  if (node.type === NodeTypes.SIMPLE_EXPRESSION) {
+    parseFilter(node, context)
+  } else {
+    for (let i = 0; i < node.children.length; i++) {
+      const child = node.children[i]
+      if (typeof child !== 'object') continue
+      if (child.type === NodeTypes.SIMPLE_EXPRESSION) {
+        parseFilter(child, context)
+      } else if (child.type === NodeTypes.COMPOUND_EXPRESSION) {
+        rewriteFilter(node, context)
+      } else if (child.type === NodeTypes.INTERPOLATION) {
+        rewriteFilter(child.content, context)
+      }
+    }
+  }
+}
+
+function parseFilter(node: SimpleExpressionNode, context: TransformContext) {
+  const exp = node.content
+  let inSingle = false
+  let inDouble = false
+  let inTemplateString = false
+  let inRegex = false
+  let curly = 0
+  let square = 0
+  let paren = 0
+  let lastFilterIndex = 0
+  let c,
+    prev,
+    i: number,
+    expression,
+    filters: string[] = []
+
+  for (i = 0; i < exp.length; i++) {
+    prev = c
+    c = exp.charCodeAt(i)
+    if (inSingle) {
+      if (c === 0x27 && prev !== 0x5c) inSingle = false
+    } else if (inDouble) {
+      if (c === 0x22 && prev !== 0x5c) inDouble = false
+    } else if (inTemplateString) {
+      if (c === 0x60 && prev !== 0x5c) inTemplateString = false
+    } else if (inRegex) {
+      if (c === 0x2f && prev !== 0x5c) inRegex = false
+    } else if (
+      c === 0x7c && // pipe
+      exp.charCodeAt(i + 1) !== 0x7c &&
+      exp.charCodeAt(i - 1) !== 0x7c &&
+      !curly &&
+      !square &&
+      !paren
+    ) {
+      if (expression === undefined) {
+        // first filter, end of expression
+        lastFilterIndex = i + 1
+        expression = exp.slice(0, i).trim()
+      } else {
+        pushFilter()
+      }
+    } else {
+      switch (c) {
+        case 0x22:
+          inDouble = true
+          break // "
+        case 0x27:
+          inSingle = true
+          break // '
+        case 0x60:
+          inTemplateString = true
+          break // `
+        case 0x28:
+          paren++
+          break // (
+        case 0x29:
+          paren--
+          break // )
+        case 0x5b:
+          square++
+          break // [
+        case 0x5d:
+          square--
+          break // ]
+        case 0x7b:
+          curly++
+          break // {
+        case 0x7d:
+          curly--
+          break // }
+      }
+      if (c === 0x2f) {
+        // /
+        let j = i - 1
+        let p
+        // find first non-whitespace prev char
+        for (; j >= 0; j--) {
+          p = exp.charAt(j)
+          if (p !== ' ') break
+        }
+        if (!p || !validDivisionCharRE.test(p)) {
+          inRegex = true
+        }
+      }
+    }
+  }
+
+  if (expression === undefined) {
+    expression = exp.slice(0, i).trim()
+  } else if (lastFilterIndex !== 0) {
+    pushFilter()
+  }
+
+  function pushFilter() {
+    filters.push(exp.slice(lastFilterIndex, i).trim())
+    lastFilterIndex = i + 1
+  }
+
+  if (
+    filters.length &&
+    warnDeprecation(
+      CompilerDeprecationTypes.COMPILER_FILTERS,
+      context,
+      node.loc
+    )
+  ) {
+    for (i = 0; i < filters.length; i++) {
+      expression = wrapFilter(expression, filters[i], context)
+    }
+    node.content = expression
+  }
+}
+
+function wrapFilter(
+  exp: string,
+  filter: string,
+  context: TransformContext
+): string {
+  context.helper(RESOLVE_FILTER)
+  const i = filter.indexOf('(')
+  if (i < 0) {
+    context.filters!.add(filter)
+    return `${toValidAssetId(filter, 'filter')}(${exp})`
+  } else {
+    const name = filter.slice(0, i)
+    const args = filter.slice(i + 1)
+    context.filters!.add(name)
+    return `${toValidAssetId(name, 'filter')}(${exp}${
+      args !== ')' ? ',' + args : args
+    }`
+  }
+}
diff --git a/packages/compiler-core/src/compile.ts b/packages/compiler-core/src/compile.ts
index e3b6260b..78a6cb62 100644
--- a/packages/compiler-core/src/compile.ts
+++ b/packages/compiler-core/src/compile.ts
@@ -15,6 +15,7 @@ import { trackSlotScopes, trackVForSlotScopes } from './transforms/vSlot'
 import { transformText } from './transforms/transformText'
 import { transformOnce } from './transforms/vOnce'
 import { transformModel } from './transforms/vModel'
+import { transformFilter } from './compat/transformFilter'
 import { defaultOnError, createCompilerError, ErrorCodes } from './errors'
 
 export type TransformPreset = [
@@ -30,6 +31,7 @@ export function getBaseTransformPreset(
       transformOnce,
       transformIf,
       transformFor,
+      ...(__COMPAT__ ? [transformFilter] : []),
       ...(!__BROWSER__ && prefixIdentifiers
         ? [
             // order is important
diff --git a/packages/compiler-core/src/errors.ts b/packages/compiler-core/src/errors.ts
index a3419c88..6dfa62cb 100644
--- a/packages/compiler-core/src/errors.ts
+++ b/packages/compiler-core/src/errors.ts
@@ -1,7 +1,7 @@
 import { SourceLocation } from './ast'
 
 export interface CompilerError extends SyntaxError {
-  code: number
+  code: number | string
   loc?: SourceLocation
 }
 
@@ -13,6 +13,10 @@ export function defaultOnError(error: CompilerError) {
   throw error
 }
 
+export function defaultOnWarn(msg: CompilerError) {
+  __DEV__ && console.warn(`[Vue warn] ${msg.message}`)
+}
+
 export function createCompilerError<T extends number>(
   code: T,
   loc?: SourceLocation,
@@ -87,13 +91,16 @@ export const enum ErrorCodes {
   X_CACHE_HANDLER_NOT_SUPPORTED,
   X_SCOPE_ID_NOT_SUPPORTED,
 
+  // warnings
+  X_V_IF_KEY,
+
   // Special value for higher-order compilers to pick up the last code
   // to avoid collision of error codes. This should always be kept as the last
   // item.
   __EXTEND_POINT__
 }
 
-export const errorMessages: { [code: number]: string } = {
+export const errorMessages: Record<ErrorCodes, string> = {
   // parse errors
   [ErrorCodes.ABRUPT_CLOSING_OF_EMPTY_COMMENT]: 'Illegal comment.',
   [ErrorCodes.CDATA_IN_HTML_CONTENT]:
@@ -124,6 +131,7 @@ export const errorMessages: { [code: number]: string } = {
     "Attribute name cannot start with '='.",
   [ErrorCodes.UNEXPECTED_QUESTION_MARK_INSTEAD_OF_TAG_NAME]:
     "'<?' is allowed only in XML context.",
+  [ErrorCodes.UNEXPECTED_NULL_CHARACTER]: `Unexpected null cahracter.`,
   [ErrorCodes.UNEXPECTED_SOLIDUS_IN_TAG]: "Illegal '/' in tags.",
 
   // Vue-specific parse errors
@@ -164,5 +172,13 @@ export const errorMessages: { [code: number]: string } = {
   [ErrorCodes.X_PREFIX_ID_NOT_SUPPORTED]: `"prefixIdentifiers" option is not supported in this build of compiler.`,
   [ErrorCodes.X_MODULE_MODE_NOT_SUPPORTED]: `ES module mode is not supported in this build of compiler.`,
   [ErrorCodes.X_CACHE_HANDLER_NOT_SUPPORTED]: `"cacheHandlers" option is only supported when the "prefixIdentifiers" option is enabled.`,
-  [ErrorCodes.X_SCOPE_ID_NOT_SUPPORTED]: `"scopeId" option is only supported in module mode.`
+  [ErrorCodes.X_SCOPE_ID_NOT_SUPPORTED]: `"scopeId" option is only supported in module mode.`,
+
+  // warnings
+  [ErrorCodes.X_V_IF_KEY]:
+    `unnecessary key usage on v-if/else branches. ` +
+    `Vue will automatically generate unique keys for each branch.`,
+
+  // just to fullfill types
+  [ErrorCodes.__EXTEND_POINT__]: ``
 }
diff --git a/packages/compiler-core/src/index.ts b/packages/compiler-core/src/index.ts
index 03388b1f..303de3af 100644
--- a/packages/compiler-core/src/index.ts
+++ b/packages/compiler-core/src/index.ts
@@ -57,3 +57,10 @@ export {
 } from './transforms/transformElement'
 export { processSlotOutlet } from './transforms/transformSlotOutlet'
 export { generateCodeFrame } from '@vue/shared'
+
+// v2 compat only
+export {
+  checkCompatEnabled,
+  warnDeprecation,
+  CompilerDeprecationTypes
+} from './compat/compatConfig'
diff --git a/packages/compiler-core/src/options.ts b/packages/compiler-core/src/options.ts
index 1b5450de..a387c167 100644
--- a/packages/compiler-core/src/options.ts
+++ b/packages/compiler-core/src/options.ts
@@ -6,9 +6,17 @@ import {
   DirectiveTransform,
   TransformContext
 } from './transform'
+import { CompilerCompatOptions } from './compat/compatConfig'
 import { ParserPlugin } from '@babel/parser'
 
-export interface ParserOptions {
+export interface ErrorHandlingOptions {
+  onWarn?: (warning: CompilerError) => void
+  onError?: (error: CompilerError) => void
+}
+
+export interface ParserOptions
+  extends ErrorHandlingOptions,
+    CompilerCompatOptions {
   /**
    * e.g. platform native elements, e.g. `<div>` for browsers
    */
@@ -44,11 +52,14 @@ export interface ParserOptions {
    * @default ['{{', '}}']
    */
   delimiters?: [string, string]
+  /**
+   * Whitespace handling strategy
+   */
+  whitespace?: 'preserve' | 'condense'
   /**
    * Only needed for DOM compilers
    */
   decodeEntities?: (rawText: string, asAttr: boolean) => string
-  onError?: (error: CompilerError) => void
   /**
    * Keep comments in the templates AST, even in production
    */
@@ -138,7 +149,10 @@ interface SharedTransformCodegenOptions {
   filename?: string
 }
 
-export interface TransformOptions extends SharedTransformCodegenOptions {
+export interface TransformOptions
+  extends SharedTransformCodegenOptions,
+    ErrorHandlingOptions,
+    CompilerCompatOptions {
   /**
    * An array of node transforms to be applied to every AST node.
    */
@@ -213,7 +227,6 @@ export interface TransformOptions extends SharedTransformCodegenOptions {
    * needed to render inline CSS variables on component root
    */
   ssrCssVars?: string
-  onError?: (error: CompilerError) => void
 }
 
 export interface CodegenOptions extends SharedTransformCodegenOptions {
diff --git a/packages/compiler-core/src/parse.ts b/packages/compiler-core/src/parse.ts
index 77e91935..784d85fb 100644
--- a/packages/compiler-core/src/parse.ts
+++ b/packages/compiler-core/src/parse.ts
@@ -1,6 +1,11 @@
-import { ParserOptions } from './options'
+import { ErrorHandlingOptions, ParserOptions } from './options'
 import { NO, isArray, makeMap, extend } from '@vue/shared'
-import { ErrorCodes, createCompilerError, defaultOnError } from './errors'
+import {
+  ErrorCodes,
+  createCompilerError,
+  defaultOnError,
+  defaultOnWarn
+} from './errors'
 import {
   assert,
   advancePositionWithMutation,
@@ -25,8 +30,18 @@ import {
   createRoot,
   ConstantTypes
 } from './ast'
+import {
+  checkCompatEnabled,
+  CompilerCompatOptions,
+  CompilerDeprecationTypes,
+  warnDeprecation
+} from './compat/compatConfig'
 
-type OptionalOptions = 'isNativeTag' | 'isBuiltInComponent'
+type OptionalOptions =
+  | 'whitespace'
+  | 'isNativeTag'
+  | 'isBuiltInComponent'
+  | keyof CompilerCompatOptions
 type MergedParserOptions = Omit<Required<ParserOptions>, OptionalOptions> &
   Pick<ParserOptions, OptionalOptions>
 type AttributeValue =
@@ -59,6 +74,7 @@ export const defaultParserOptions: MergedParserOptions = {
   decodeEntities: (rawText: string): string =>
     rawText.replace(decodeRE, (_, p1) => decodeMap[p1]),
   onError: defaultOnError,
+  onWarn: defaultOnWarn,
   comments: false
 }
 
@@ -80,6 +96,7 @@ export interface ParserContext {
   column: number
   inPre: boolean // HTML <pre> tag, preserve whitespaces
   inVPre: boolean // v-pre, do not process directives and interpolations
+  onWarn: NonNullable<ErrorHandlingOptions['onWarn']>
 }
 
 export function baseParse(
@@ -111,7 +128,8 @@ function createParserContext(
     originalSource: content,
     source: content,
     inPre: false,
-    inVPre: false
+    inVPre: false,
+    onWarn: options.onWarn
   }
 }
 
@@ -202,38 +220,39 @@ function parseChildren(
     }
   }
 
-  // Whitespace management for more efficient output
-  // (same as v2 whitespace: 'condense')
+  // Whitespace handling strategy like v2
   let removedWhitespace = false
   if (mode !== TextModes.RAWTEXT && mode !== TextModes.RCDATA) {
+    const preserve = context.options.whitespace === 'preserve'
     for (let i = 0; i < nodes.length; i++) {
       const node = nodes[i]
       if (!context.inPre && node.type === NodeTypes.TEXT) {
         if (!/[^\t\r\n\f ]/.test(node.content)) {
           const prev = nodes[i - 1]
           const next = nodes[i + 1]
-          // If:
+          // Remove if:
           // - the whitespace is the first or last node, or:
-          // - the whitespace is adjacent to a comment, or:
-          // - the whitespace is between two elements AND contains newline
-          // Then the whitespace is ignored.
+          // - (condense mode) the whitespace is adjacent to a comment, or:
+          // - (condense mode) the whitespace is between two elements AND contains newline
           if (
             !prev ||
             !next ||
-            prev.type === NodeTypes.COMMENT ||
-            next.type === NodeTypes.COMMENT ||
-            (prev.type === NodeTypes.ELEMENT &&
-              next.type === NodeTypes.ELEMENT &&
-              /[\r\n]/.test(node.content))
+            (!preserve &&
+              (prev.type === NodeTypes.COMMENT ||
+                next.type === NodeTypes.COMMENT ||
+                (prev.type === NodeTypes.ELEMENT &&
+                  next.type === NodeTypes.ELEMENT &&
+                  /[\r\n]/.test(node.content))))
           ) {
             removedWhitespace = true
             nodes[i] = null as any
           } else {
-            // Otherwise, condensed consecutive whitespace inside the text
-            // down to a single space
+            // Otherwise, the whitespace is condensed into a single space
             node.content = ' '
           }
-        } else {
+        } else if (!preserve) {
+          // in condense mode, consecutive whitespaces in text are condensed
+          // down to a single space.
           node.content = node.content.replace(/[\t\r\n\f ]+/g, ' ')
         }
       }
@@ -389,6 +408,27 @@ function parseElement(
   const children = parseChildren(context, mode, ancestors)
   ancestors.pop()
 
+  // 2.x inline-template compat
+  if (__COMPAT__) {
+    const inlineTemplateProp = element.props.find(
+      p => p.type === NodeTypes.ATTRIBUTE && p.name === 'inline-template'
+    ) as AttributeNode
+    if (
+      inlineTemplateProp &&
+      checkCompatEnabled(
+        CompilerDeprecationTypes.COMPILER_INLINE_TEMPLATE,
+        context,
+        inlineTemplateProp.loc
+      )
+    ) {
+      inlineTemplateProp.value!.content = getSelection(
+        context,
+        element.loc.end
+      ).source
+      console.log(inlineTemplateProp)
+    }
+  }
+
   element.children = children
 
   // End tag.
@@ -427,11 +467,21 @@ const isSpecialTemplateDirective = /*#__PURE__*/ makeMap(
 /**
  * Parse a tag (E.g. `<div id=a>`) with that type (start tag or end tag).
  */
+function parseTag(
+  context: ParserContext,
+  type: TagType.Start,
+  parent: ElementNode | undefined
+): ElementNode
+function parseTag(
+  context: ParserContext,
+  type: TagType.End,
+  parent: ElementNode | undefined
+): void
 function parseTag(
   context: ParserContext,
   type: TagType,
   parent: ElementNode | undefined
-): ElementNode {
+): ElementNode | undefined {
   __TEST__ && assert(/^<\/?[a-z]/i.test(context.source))
   __TEST__ &&
     assert(
@@ -461,6 +511,7 @@ function parseTag(
 
   // check v-pre
   if (
+    type === TagType.Start &&
     !context.inVPre &&
     props.some(p => p.type === NodeTypes.DIRECTIVE && p.name === 'pre')
   ) {
@@ -484,12 +535,58 @@ function parseTag(
     advanceBy(context, isSelfClosing ? 2 : 1)
   }
 
+  if (type === TagType.End) {
+    return
+  }
+
+  // 2.x deprecation checks
+  if (__COMPAT__ && __DEV__ && !__TEST__) {
+    let hasIf = false
+    let hasFor = false
+    for (let i = 0; i < props.length; i++) {
+      const p = props[i]
+      if (p.type === NodeTypes.DIRECTIVE) {
+        if (p.name === 'if') {
+          hasIf = true
+        } else if (p.name === 'for') {
+          hasFor = true
+        }
+      }
+      if (hasIf && hasFor) {
+        warnDeprecation(
+          CompilerDeprecationTypes.COMPILER_V_IF_V_FOR_PRECEDENCE,
+          context,
+          getSelection(context, start)
+        )
+      }
+    }
+  }
+
   let tagType = ElementTypes.ELEMENT
   const options = context.options
   if (!context.inVPre && !options.isCustomElement(tag)) {
-    const hasVIs = props.some(
-      p => p.type === NodeTypes.DIRECTIVE && p.name === 'is'
-    )
+    const hasVIs = props.some(p => {
+      if (p.name !== 'is') return
+      // v-is="xxx" (TODO: deprecate)
+      if (p.type === NodeTypes.DIRECTIVE) {
+        return true
+      }
+      // is="vue:xxx"
+      if (p.value && p.value.content.startsWith('vue:')) {
+        return true
+      }
+      // in compat mode, any is usage is considered a component
+      if (
+        __COMPAT__ &&
+        checkCompatEnabled(
+          CompilerDeprecationTypes.COMPILER_IS_ON_ELEMENT,
+          context,
+          p.loc
+        )
+      ) {
+        return true
+      }
+    })
     if (options.isNativeTag && !hasVIs) {
       if (!options.isNativeTag(tag)) tagType = ElementTypes.COMPONENT
     } else if (
@@ -506,11 +603,10 @@ function parseTag(
       tagType = ElementTypes.SLOT
     } else if (
       tag === 'template' &&
-      props.some(p => {
-        return (
+      props.some(
+        p =>
           p.type === NodeTypes.DIRECTIVE && isSpecialTemplateDirective(p.name)
-        )
-      })
+      )
     ) {
       tagType = ElementTypes.TEMPLATE
     }
@@ -615,10 +711,9 @@ function parseAttribute(
       name
     )!
 
-    const dirName =
+    let dirName =
       match[1] ||
       (startsWith(name, ':') ? 'bind' : startsWith(name, '@') ? 'on' : 'slot')
-
     let arg: ExpressionNode | undefined
 
     if (match[2]) {
@@ -673,6 +768,32 @@ function parseAttribute(
       valueLoc.source = valueLoc.source.slice(1, -1)
     }
 
+    const modifiers = match[3] ? match[3].substr(1).split('.') : []
+
+    // 2.x compat v-bind:foo.sync -> v-model:foo
+    if (__COMPAT__ && dirName === 'bind' && arg) {
+      if (
+        modifiers.includes('sync') &&
+        checkCompatEnabled(
+          CompilerDeprecationTypes.COMPILER_V_BIND_SYNC,
+          context,
+          loc,
+          arg.loc.source
+        )
+      ) {
+        dirName = 'model'
+        modifiers.splice(modifiers.indexOf('sync'), 1)
+      }
+
+      if (__DEV__ && modifiers.includes('prop')) {
+        checkCompatEnabled(
+          CompilerDeprecationTypes.COMPILER_V_BIND_PROP,
+          context,
+          loc
+        )
+      }
+    }
+
     return {
       type: NodeTypes.DIRECTIVE,
       name: dirName,
@@ -686,7 +807,7 @@ function parseAttribute(
         loc: value.loc
       },
       arg,
-      modifiers: match[3] ? match[3].substr(1).split('.') : [],
+      modifiers,
       loc
     }
   }
diff --git a/packages/compiler-core/src/runtimeHelpers.ts b/packages/compiler-core/src/runtimeHelpers.ts
index f40c94c3..94ce1ebc 100644
--- a/packages/compiler-core/src/runtimeHelpers.ts
+++ b/packages/compiler-core/src/runtimeHelpers.ts
@@ -14,6 +14,7 @@ export const RESOLVE_DYNAMIC_COMPONENT = Symbol(
   __DEV__ ? `resolveDynamicComponent` : ``
 )
 export const RESOLVE_DIRECTIVE = Symbol(__DEV__ ? `resolveDirective` : ``)
+export const RESOLVE_FILTER = Symbol(__DEV__ ? `resolveFilter` : ``)
 export const WITH_DIRECTIVES = Symbol(__DEV__ ? `withDirectives` : ``)
 export const RENDER_LIST = Symbol(__DEV__ ? `renderList` : ``)
 export const RENDER_SLOT = Symbol(__DEV__ ? `renderSlot` : ``)
@@ -50,6 +51,7 @@ export const helperNameMap: any = {
   [RESOLVE_COMPONENT]: `resolveComponent`,
   [RESOLVE_DYNAMIC_COMPONENT]: `resolveDynamicComponent`,
   [RESOLVE_DIRECTIVE]: `resolveDirective`,
+  [RESOLVE_FILTER]: `resolveFilter`,
   [WITH_DIRECTIVES]: `withDirectives`,
   [RENDER_LIST]: `renderList`,
   [RENDER_SLOT]: `renderSlot`,
diff --git a/packages/compiler-core/src/transform.ts b/packages/compiler-core/src/transform.ts
index 9dd5b8be..add7ea13 100644
--- a/packages/compiler-core/src/transform.ts
+++ b/packages/compiler-core/src/transform.ts
@@ -28,7 +28,7 @@ import {
   capitalize,
   camelize
 } from '@vue/shared'
-import { defaultOnError } from './errors'
+import { defaultOnError, defaultOnWarn } from './errors'
 import {
   TO_DISPLAY_STRING,
   FRAGMENT,
@@ -40,6 +40,7 @@ import {
 } from './runtimeHelpers'
 import { isVSlot } from './utils'
 import { hoistStatic, isSingleElementRoot } from './transforms/hoistStatic'
+import { CompilerCompatOptions } from './compat/compatConfig'
 
 // There are two types of transforms:
 //
@@ -83,7 +84,10 @@ export interface ImportItem {
 }
 
 export interface TransformContext
-  extends Required<Omit<TransformOptions, 'filename'>> {
+  extends Required<
+      Omit<TransformOptions, 'filename' | keyof CompilerCompatOptions>
+    >,
+    CompilerCompatOptions {
   selfName: string | null
   root: RootNode
   helpers: Map<symbol, number>
@@ -114,6 +118,9 @@ export interface TransformContext
   hoist(exp: JSChildNode): SimpleExpressionNode
   cache<T extends JSChildNode>(exp: T, isVNode?: boolean): CacheExpression | T
   constantCache: Map<TemplateChildNode, ConstantTypes>
+
+  // 2.x Compat only
+  filters?: Set<string>
 }
 
 export function createTransformContext(
@@ -136,7 +143,9 @@ export function createTransformContext(
     bindingMetadata = EMPTY_OBJ,
     inline = false,
     isTS = false,
-    onError = defaultOnError
+    onError = defaultOnError,
+    onWarn = defaultOnWarn,
+    compatConfig
   }: TransformOptions
 ): TransformContext {
   const nameMatch = filename.replace(/\?.*$/, '').match(/([^/\\]+)\.\w+$/)
@@ -160,6 +169,8 @@ export function createTransformContext(
     inline,
     isTS,
     onError,
+    onWarn,
+    compatConfig,
 
     // state
     root,
@@ -281,6 +292,10 @@ export function createTransformContext(
     }
   }
 
+  if (__COMPAT__) {
+    context.filters = new Set()
+  }
+
   function addId(id: string) {
     const { identifiers } = context
     if (identifiers[id] === undefined) {
@@ -313,6 +328,10 @@ export function transform(root: RootNode, options: TransformOptions) {
   root.hoists = context.hoists
   root.temps = context.temps
   root.cached = context.cached
+
+  if (__COMPAT__) {
+    root.filters = [...context.filters!]
+  }
 }
 
 function createRootCodegen(root: RootNode, context: TransformContext) {
diff --git a/packages/compiler-core/src/transforms/transformElement.ts b/packages/compiler-core/src/transforms/transformElement.ts
index 113b03dd..bf040467 100644
--- a/packages/compiler-core/src/transforms/transformElement.ts
+++ b/packages/compiler-core/src/transforms/transformElement.ts
@@ -41,7 +41,8 @@ import {
   TELEPORT,
   KEEP_ALIVE,
   SUSPENSE,
-  UNREF
+  UNREF,
+  FRAGMENT
 } from '../runtimeHelpers'
 import {
   getInnerRange,
@@ -55,6 +56,11 @@ import {
 import { buildSlots } from './vSlot'
 import { getConstantType } from './hoistStatic'
 import { BindingTypes } from '../options'
+import {
+  checkCompatEnabled,
+  CompilerDeprecationTypes,
+  isCompatEnabled
+} from '../compat/compatConfig'
 
 // some directive transforms (e.g. v-model) may return a symbol for runtime
 // import, which should be used instead of a resolveDirective call.
@@ -82,9 +88,23 @@ export const transformElement: NodeTransform = (node, context) => {
 
     // The goal of the transform is to create a codegenNode implementing the
     // VNodeCall interface.
-    const vnodeTag = isComponent
+    let vnodeTag = isComponent
       ? resolveComponentType(node as ComponentNode, context)
       : `"${tag}"`
+
+    // 2.x <template> with no directives compat
+    if (
+      __COMPAT__ &&
+      tag === 'template' &&
+      checkCompatEnabled(
+        CompilerDeprecationTypes.COMPILER_NATIVE_TEMPLATE,
+        context,
+        node.loc
+      )
+    ) {
+      vnodeTag = context.helper(FRAGMENT)
+    }
+
     const isDynamicComponent =
       isObject(vnodeTag) && vnodeTag.callee === RESOLVE_DYNAMIC_COMPONENT
 
@@ -230,21 +250,29 @@ export function resolveComponentType(
   context: TransformContext,
   ssr = false
 ) {
-  const { tag } = node
+  let { tag } = node
 
   // 1. dynamic component
-  const isProp = isComponentTag(tag)
-    ? findProp(node, 'is')
-    : findDir(node, 'is')
+  const isExplicitDynamic = isComponentTag(tag)
+  const isProp =
+    findProp(node, 'is') || (!isExplicitDynamic && findDir(node, 'is'))
   if (isProp) {
-    const exp =
-      isProp.type === NodeTypes.ATTRIBUTE
-        ? isProp.value && createSimpleExpression(isProp.value.content, true)
-        : isProp.exp
-    if (exp) {
-      return createCallExpression(context.helper(RESOLVE_DYNAMIC_COMPONENT), [
-        exp
-      ])
+    if (!isExplicitDynamic && isProp.type === NodeTypes.ATTRIBUTE) {
+      // <button is="vue:xxx">
+      // if not <component>, only is value that starts with "vue:" will be
+      // treated as component by the parse phase and reach here, unless it's
+      // compat mode where all is values are considered components
+      tag = isProp.value!.content.replace(/^vue:/, '')
+    } else {
+      const exp =
+        isProp.type === NodeTypes.ATTRIBUTE
+          ? isProp.value && createSimpleExpression(isProp.value.content, true)
+          : isProp.exp
+      if (exp) {
+        return createCallExpression(context.helper(RESOLVE_DYNAMIC_COMPONENT), [
+          exp
+        ])
+      }
     }
   }
 
@@ -416,8 +444,11 @@ export function buildProps(
           isStatic = false
         }
       }
-      // skip :is on <component>
-      if (name === 'is' && isComponentTag(tag)) {
+      // skip is on <component>, or is="vue:xxx"
+      if (
+        name === 'is' &&
+        (isComponentTag(tag) || (value && value.content.startsWith('vue:')))
+      ) {
         continue
       }
       properties.push(
@@ -437,8 +468,8 @@ export function buildProps(
     } else {
       // directives
       const { name, arg, exp, loc } = prop
-      const isBind = name === 'bind'
-      const isOn = name === 'on'
+      const isVBind = name === 'bind'
+      const isVOn = name === 'on'
 
       // skip v-slot - it is handled by its dedicated transform.
       if (name === 'slot') {
@@ -456,17 +487,17 @@ export function buildProps(
       // skip v-is and :is on <component>
       if (
         name === 'is' ||
-        (isBind && isComponentTag(tag) && isBindKey(arg, 'is'))
+        (isVBind && isComponentTag(tag) && isBindKey(arg, 'is'))
       ) {
         continue
       }
       // skip v-on in SSR compilation
-      if (isOn && ssr) {
+      if (isVOn && ssr) {
         continue
       }
 
       // special case for v-bind and v-on with no argument
-      if (!arg && (isBind || isOn)) {
+      if (!arg && (isVBind || isVOn)) {
         hasDynamicKeys = true
         if (exp) {
           if (properties.length) {
@@ -475,7 +506,50 @@ export function buildProps(
             )
             properties = []
           }
-          if (isBind) {
+          if (isVBind) {
+            if (__COMPAT__) {
+              // 2.x v-bind object order compat
+              if (__DEV__) {
+                const hasOverridableKeys = mergeArgs.some(arg => {
+                  if (arg.type === NodeTypes.JS_OBJECT_EXPRESSION) {
+                    return arg.properties.some(({ key }) => {
+                      if (
+                        key.type !== NodeTypes.SIMPLE_EXPRESSION ||
+                        !key.isStatic
+                      ) {
+                        return true
+                      }
+                      return (
+                        key.content !== 'class' &&
+                        key.content !== 'style' &&
+                        !isOn(key.content)
+                      )
+                    })
+                  } else {
+                    // dynamic expression
+                    return true
+                  }
+                })
+                if (hasOverridableKeys) {
+                  checkCompatEnabled(
+                    CompilerDeprecationTypes.COMPILER_V_BIND_OBJECT_ORDER,
+                    context,
+                    loc
+                  )
+                }
+              }
+
+              if (
+                isCompatEnabled(
+                  CompilerDeprecationTypes.COMPILER_V_BIND_OBJECT_ORDER,
+                  context
+                )
+              ) {
+                mergeArgs.unshift(exp)
+                continue
+              }
+            }
+
             mergeArgs.push(exp)
           } else {
             // v-on="obj" -> toHandlers(obj)
@@ -489,7 +563,7 @@ export function buildProps(
         } else {
           context.onError(
             createCompilerError(
-              isBind
+              isVBind
                 ? ErrorCodes.X_V_BIND_NO_EXPRESSION
                 : ErrorCodes.X_V_ON_NO_EXPRESSION,
               loc
@@ -516,6 +590,25 @@ export function buildProps(
         runtimeDirectives.push(prop)
       }
     }
+
+    if (
+      __COMPAT__ &&
+      prop.type === NodeTypes.ATTRIBUTE &&
+      prop.name === 'ref' &&
+      context.scopes.vFor > 0 &&
+      checkCompatEnabled(
+        CompilerDeprecationTypes.COMPILER_V_FOR_REF,
+        context,
+        prop.loc
+      )
+    ) {
+      properties.push(
+        createObjectProperty(
+          createSimpleExpression('refInFor', true),
+          createSimpleExpression('true', false)
+        )
+      )
+    }
   }
 
   let propsExpression: PropsExpression | undefined = undefined
diff --git a/packages/compiler-core/src/transforms/transformExpression.ts b/packages/compiler-core/src/transforms/transformExpression.ts
index 1b746111..4ebfbbe7 100644
--- a/packages/compiler-core/src/transforms/transformExpression.ts
+++ b/packages/compiler-core/src/transforms/transformExpression.ts
@@ -254,6 +254,11 @@ export function processExpression(
       parent && parentStack.push(parent)
       if (node.type === 'Identifier') {
         if (!isDuplicate(node)) {
+          // v2 wrapped filter call
+          if (__COMPAT__ && node.name.startsWith('_filter_')) {
+            return
+          }
+
           const needPrefix = shouldPrefix(node, parent!, parentStack)
           if (!knownIds[node.name] && needPrefix) {
             if (isStaticProperty(parent!) && parent.shorthand) {
diff --git a/packages/compiler-core/src/transforms/transformText.ts b/packages/compiler-core/src/transforms/transformText.ts
index 3576e0e2..a8b65c96 100644
--- a/packages/compiler-core/src/transforms/transformText.ts
+++ b/packages/compiler-core/src/transforms/transformText.ts
@@ -63,7 +63,11 @@ export const transformText: NodeTransform = (node, context) => {
         (children.length === 1 &&
           (node.type === NodeTypes.ROOT ||
             (node.type === NodeTypes.ELEMENT &&
-              node.tagType === ElementTypes.ELEMENT)))
+              node.tagType === ElementTypes.ELEMENT &&
+              // in compat mode, <template> tags with no special directives
+              // will be rendered as a fragment so its children must be
+              // converted into vnodes.
+              !(__COMPAT__ && node.tag === 'template'))))
       ) {
         return
       }
diff --git a/packages/compiler-core/src/transforms/vBind.ts b/packages/compiler-core/src/transforms/vBind.ts
index 0d31a266..c1faed1a 100644
--- a/packages/compiler-core/src/transforms/vBind.ts
+++ b/packages/compiler-core/src/transforms/vBind.ts
@@ -7,7 +7,7 @@ import { CAMELIZE } from '../runtimeHelpers'
 // v-bind without arg is handled directly in ./transformElements.ts due to it affecting
 // codegen for the entire props object. This transform here is only for v-bind
 // *with* args.
-export const transformBind: DirectiveTransform = (dir, node, context) => {
+export const transformBind: DirectiveTransform = (dir, _node, context) => {
   const { exp, modifiers, loc } = dir
   const arg = dir.arg!
 
diff --git a/packages/compiler-core/src/transforms/vIf.ts b/packages/compiler-core/src/transforms/vIf.ts
index 384b6612..9ba84f8b 100644
--- a/packages/compiler-core/src/transforms/vIf.ts
+++ b/packages/compiler-core/src/transforms/vIf.ts
@@ -110,7 +110,7 @@ export function processIf(
   }
 
   if (dir.name === 'if') {
-    const branch = createIfBranch(node, dir)
+    const branch = createIfBranch(node, dir, context)
     const ifNode: IfNode = {
       type: NodeTypes.IF,
       loc: node.loc,
@@ -145,7 +145,7 @@ export function processIf(
       if (sibling && sibling.type === NodeTypes.IF) {
         // move the node to the if node's branches
         context.removeNode()
-        const branch = createIfBranch(node, dir)
+        const branch = createIfBranch(node, dir, context)
         if (__DEV__ && comments.length) {
           branch.children = [...comments, ...branch.children]
         }
@@ -187,7 +187,16 @@ export function processIf(
   }
 }
 
-function createIfBranch(node: ElementNode, dir: DirectiveNode): IfBranchNode {
+function createIfBranch(
+  node: ElementNode,
+  dir: DirectiveNode,
+  context: TransformContext
+): IfBranchNode {
+  const userKey = findProp(node, `key`)
+  if (__DEV__ && userKey) {
+    context.onWarn(createCompilerError(ErrorCodes.X_V_IF_KEY, userKey.loc))
+  }
+
   return {
     type: NodeTypes.IF_BRANCH,
     loc: node.loc,
@@ -196,7 +205,7 @@ function createIfBranch(node: ElementNode, dir: DirectiveNode): IfBranchNode {
       node.tagType === ElementTypes.TEMPLATE && !findDir(node, 'for')
         ? node.children
         : [node],
-    userKey: findProp(node, `key`)
+    userKey
   }
 }
 
diff --git a/packages/compiler-core/src/utils.ts b/packages/compiler-core/src/utils.ts
index b65c812a..370a9ac6 100644
--- a/packages/compiler-core/src/utils.ts
+++ b/packages/compiler-core/src/utils.ts
@@ -271,7 +271,7 @@ export function injectProp(
 
 export function toValidAssetId(
   name: string,
-  type: 'component' | 'directive'
+  type: 'component' | 'directive' | 'filter'
 ): string {
   return `_${type}_${name.replace(/[^\w]/g, '_')}`
 }
diff --git a/packages/compiler-dom/package.json b/packages/compiler-dom/package.json
index 8ae4bcea..801c3b9d 100644
--- a/packages/compiler-dom/package.json
+++ b/packages/compiler-dom/package.json
@@ -14,6 +14,7 @@
   "sideEffects": false,
   "buildOptions": {
     "name": "VueCompilerDOM",
+    "compat": true,
     "formats": [
       "esm-bundler",
       "esm-browser",
diff --git a/packages/compiler-dom/src/errors.ts b/packages/compiler-dom/src/errors.ts
index d88b7ca8..159bd4ba 100644
--- a/packages/compiler-dom/src/errors.ts
+++ b/packages/compiler-dom/src/errors.ts
@@ -12,12 +12,12 @@ export interface DOMCompilerError extends CompilerError {
 export function createDOMCompilerError(
   code: DOMErrorCodes,
   loc?: SourceLocation
-): DOMCompilerError {
+) {
   return createCompilerError(
     code,
     loc,
     __DEV__ || !__BROWSER__ ? DOMErrorMessages : undefined
-  )
+  ) as DOMCompilerError
 }
 
 export const enum DOMErrorCodes {
diff --git a/packages/compiler-dom/src/transforms/vOn.ts b/packages/compiler-dom/src/transforms/vOn.ts
index 50c9bf75..7ff4b7b6 100644
--- a/packages/compiler-dom/src/transforms/vOn.ts
+++ b/packages/compiler-dom/src/transforms/vOn.ts
@@ -8,7 +8,9 @@ import {
   createCompoundExpression,
   ExpressionNode,
   SimpleExpressionNode,
-  isStaticExp
+  isStaticExp,
+  warnDeprecation,
+  CompilerDeprecationTypes
 } from '@vue/compiler-core'
 import { V_ON_WITH_MODIFIERS, V_ON_WITH_KEYS } from '../runtimeHelpers'
 import { makeMap, capitalize } from '@vue/shared'
@@ -92,6 +94,14 @@ export const transformOn: DirectiveTransform = (dir, node, context) => {
     const { modifiers } = dir
     if (!modifiers.length) return baseResult
 
+    if (__COMPAT__ && __DEV__ && modifiers.includes('native')) {
+      warnDeprecation(
+        CompilerDeprecationTypes.COMPILER_V_ON_NATIVE,
+        context,
+        dir.loc
+      )
+    }
+
     let { key, value: handlerExp } = baseResult.props[0]
     const {
       keyModifiers,
diff --git a/packages/compiler-sfc/src/parse.ts b/packages/compiler-sfc/src/parse.ts
index df7059c1..dabd227f 100644
--- a/packages/compiler-sfc/src/parse.ts
+++ b/packages/compiler-sfc/src/parse.ts
@@ -155,6 +155,18 @@ export function parse(
             false
           ) as SFCTemplateBlock)
           templateBlock.ast = node
+
+          // warn against 2.x <template functional>
+          if (templateBlock.attrs.functional) {
+            const err = new SyntaxError(
+              `<template functional> is no longer supported in Vue 3, since ` +
+                `functional components no longer have significant performance ` +
+                `difference from stateful ones. Just use a normal <template> ` +
+                `instead.`
+            ) as CompilerError
+            err.loc = node.props.find(p => p.name === 'functional')!.loc
+            errors.push(err)
+          }
         } else {
           errors.push(createDuplicateBlockError(node))
         }
diff --git a/packages/compiler-ssr/src/errors.ts b/packages/compiler-ssr/src/errors.ts
index a1e057a7..5030a56d 100644
--- a/packages/compiler-ssr/src/errors.ts
+++ b/packages/compiler-ssr/src/errors.ts
@@ -12,8 +12,8 @@ export interface SSRCompilerError extends CompilerError {
 export function createSSRCompilerError(
   code: SSRErrorCodes,
   loc?: SourceLocation
-): SSRCompilerError {
-  return createCompilerError(code, loc, SSRErrorMessages)
+) {
+  return createCompilerError(code, loc, SSRErrorMessages) as SSRCompilerError
 }
 
 export const enum SSRErrorCodes {
diff --git a/packages/global.d.ts b/packages/global.d.ts
index 8c6c57d8..72ab2fe9 100644
--- a/packages/global.d.ts
+++ b/packages/global.d.ts
@@ -8,6 +8,7 @@ declare var __ESM_BROWSER__: boolean
 declare var __NODE_JS__: boolean
 declare var __COMMIT__: string
 declare var __VERSION__: string
+declare var __COMPAT__: boolean
 
 // Feature flags
 declare var __FEATURE_OPTIONS_API__: boolean
diff --git a/packages/runtime-core/__tests__/apiCreateApp.spec.ts b/packages/runtime-core/__tests__/apiCreateApp.spec.ts
index 82fd44d4..bed6c0b1 100644
--- a/packages/runtime-core/__tests__/apiCreateApp.spec.ts
+++ b/packages/runtime-core/__tests__/apiCreateApp.spec.ts
@@ -481,4 +481,7 @@ describe('api: createApp', () => {
     app.mount(root)
     expect(serializeInner(root)).toBe('hello')
   })
+
+  // config.compilerOptions is tested in packages/vue since it is only
+  // supported in the full build.
 })
diff --git a/packages/runtime-core/__tests__/componentPublicInstance.spec.ts b/packages/runtime-core/__tests__/componentPublicInstance.spec.ts
index 4a4b56a8..4411771a 100644
--- a/packages/runtime-core/__tests__/componentPublicInstance.spec.ts
+++ b/packages/runtime-core/__tests__/componentPublicInstance.spec.ts
@@ -6,7 +6,7 @@ import {
   createApp,
   shallowReadonly
 } from '@vue/runtime-test'
-import { ComponentInternalInstance } from '../src/component'
+import { ComponentInternalInstance, ComponentOptions } from '../src/component'
 
 describe('component: proxy', () => {
   test('data', () => {
@@ -93,7 +93,7 @@ describe('component: proxy', () => {
     expect(instanceProxy.$root).toBe(instance!.root.proxy)
     expect(instanceProxy.$emit).toBe(instance!.emit)
     expect(instanceProxy.$el).toBe(instance!.vnode.el)
-    expect(instanceProxy.$options).toBe(instance!.type)
+    expect(instanceProxy.$options).toBe(instance!.type as ComponentOptions)
     expect(() => (instanceProxy.$data = {})).toThrow(TypeError)
     expect(`Attempting to mutate public property "$data"`).toHaveBeenWarned()
 
diff --git a/packages/runtime-core/__tests__/componentSlots.spec.ts b/packages/runtime-core/__tests__/componentSlots.spec.ts
index ecb548f2..9248a8e6 100644
--- a/packages/runtime-core/__tests__/componentSlots.spec.ts
+++ b/packages/runtime-core/__tests__/componentSlots.spec.ts
@@ -82,7 +82,7 @@ describe('component: slots', () => {
     expect(slots.default()).toMatchObject([normalizeVNode(h('span'))])
   })
 
-  test('updateSlots: instance.slots should be update correctly (when slotType is number)', async () => {
+  test('updateSlots: instance.slots should be updated correctly (when slotType is number)', async () => {
     const flag1 = ref(true)
 
     let instance: any
@@ -124,7 +124,7 @@ describe('component: slots', () => {
     expect(instance.slots).toHaveProperty('two')
   })
 
-  test('updateSlots: instance.slots should be update correctly (when slotType is null)', async () => {
+  test('updateSlots: instance.slots should be updated correctly (when slotType is null)', async () => {
     const flag1 = ref(true)
 
     let instance: any
diff --git a/packages/runtime-core/src/apiCreateApp.ts b/packages/runtime-core/src/apiCreateApp.ts
index 92cce584..e6fa3299 100644
--- a/packages/runtime-core/src/apiCreateApp.ts
+++ b/packages/runtime-core/src/apiCreateApp.ts
@@ -4,17 +4,20 @@ import {
   validateComponentName,
   Component
 } from './component'
-import { ComponentOptions } from './componentOptions'
+import { ComponentOptions, RuntimeCompilerOptions } from './componentOptions'
 import { ComponentPublicInstance } from './componentPublicInstance'
 import { Directive, validateDirectiveName } from './directives'
 import { RootRenderFunction } from './renderer'
 import { InjectionKey } from './apiInject'
-import { isFunction, NO, isObject } from '@vue/shared'
 import { warn } from './warning'
 import { createVNode, cloneVNode, VNode } from './vnode'
 import { RootHydrateFunction } from './hydration'
 import { devtoolsInitApp, devtoolsUnmountApp } from './devtools'
+import { isFunction, NO, isObject } from '@vue/shared'
 import { version } from '.'
+import { installCompatMount } from './compat/global'
+import { installLegacyConfigProperties } from './compat/globalConfig'
+import { installGlobalFilterMethod } from './compat/filter'
 
 export interface App<HostElement = any> {
   version: string
@@ -39,6 +42,17 @@ export interface App<HostElement = any> {
   _props: Data | null
   _container: HostElement | null
   _context: AppContext
+
+  /**
+   * v2 compat only
+   */
+  filter?(name: string): Function | undefined
+  filter?(name: string, filter: Function): this
+
+  /**
+   * @internal v3 compat only
+   */
+  _createRoot?(options: ComponentOptions): ComponentPublicInstance
 }
 
 export type OptionMergeFunction = (
@@ -55,7 +69,6 @@ export interface AppConfig {
   performance: boolean
   optionMergeStrategies: Record<string, OptionMergeFunction>
   globalProperties: Record<string, any>
-  isCustomElement: (tag: string) => boolean
   errorHandler?: (
     err: unknown,
     instance: ComponentPublicInstance | null,
@@ -66,6 +79,17 @@ export interface AppConfig {
     instance: ComponentPublicInstance | null,
     trace: string
   ) => void
+
+  /**
+   * @deprecated use config.compilerOptions.isCustomElement
+   */
+  isCustomElement?: (tag: string) => boolean
+
+  /**
+   * Options to pass to @vue/compiler-dom.
+   * Only supported in runtime compiler build.
+   */
+  compilerOptions: RuntimeCompilerOptions
 }
 
 export interface AppContext {
@@ -85,6 +109,11 @@ export interface AppContext {
    * @internal
    */
   reload?: () => void
+  /**
+   * v2 compat only
+   * @internal
+   */
+  filters?: Record<string, Function>
 }
 
 type PluginInstallFunction = (app: App, ...options: any[]) => any
@@ -103,9 +132,11 @@ export function createAppContext(): AppContext {
       performance: false,
       globalProperties: {},
       optionMergeStrategies: {},
-      isCustomElement: NO,
       errorHandler: undefined,
-      warnHandler: undefined
+      warnHandler: undefined,
+      compilerOptions: {
+        isCustomElement: NO
+      }
     },
     mixins: [],
     components: {},
@@ -298,6 +329,12 @@ export function createAppAPI<HostElement>(
       }
     })
 
+    if (__COMPAT__) {
+      installCompatMount(app, context, render, hydrate)
+      installGlobalFilterMethod(app, context)
+      if (__DEV__) installLegacyConfigProperties(app.config)
+    }
+
     return app
   }
 }
diff --git a/packages/runtime-core/src/apiWatch.ts b/packages/runtime-core/src/apiWatch.ts
index 9498ced4..fa476f15 100644
--- a/packages/runtime-core/src/apiWatch.ts
+++ b/packages/runtime-core/src/apiWatch.ts
@@ -18,7 +18,8 @@ import {
   NOOP,
   remove,
   isMap,
-  isSet
+  isSet,
+  isPlainObject
 } from '@vue/shared'
 import {
   currentInstance,
@@ -33,6 +34,9 @@ import {
 } from './errorHandling'
 import { queuePostRenderEffect } from './renderer'
 import { warn } from './warning'
+import { DeprecationTypes } from './compat/compatConfig'
+import { checkCompatEnabled, isCompatEnabled } from './compat/compatConfig'
+import { ObjectWatchOptionItem } from './componentOptions'
 
 export type WatchEffect = (onInvalidate: InvalidateCbRegistrator) => void
 
@@ -217,6 +221,21 @@ function doWatch(
     __DEV__ && warnInvalidSource(source)
   }
 
+  // 2.x array mutation watch compat
+  if (__COMPAT__ && cb && !deep) {
+    const baseGetter = getter
+    getter = () => {
+      const val = baseGetter()
+      if (
+        isArray(val) &&
+        checkCompatEnabled(DeprecationTypes.WATCH_ARRAY, instance)
+      ) {
+        traverse(val)
+      }
+      return val
+    }
+  }
+
   if (cb && deep) {
     const baseGetter = getter
     getter = () => traverse(baseGetter())
@@ -254,7 +273,14 @@ function doWatch(
     if (cb) {
       // watch(source, cb)
       const newValue = runner()
-      if (deep || forceTrigger || hasChanged(newValue, oldValue)) {
+      if (
+        deep ||
+        forceTrigger ||
+        hasChanged(newValue, oldValue) ||
+        (__COMPAT__ &&
+          isArray(newValue) &&
+          isCompatEnabled(DeprecationTypes.WATCH_ARRAY, instance))
+      ) {
         // cleanup before running cb again
         if (cleanup) {
           cleanup()
@@ -329,7 +355,7 @@ function doWatch(
 export function instanceWatch(
   this: ComponentInternalInstance,
   source: string | Function,
-  cb: WatchCallback,
+  value: WatchCallback | ObjectWatchOptionItem,
   options?: WatchOptions
 ): WatchStopHandle {
   const publicThis = this.proxy as any
@@ -338,6 +364,13 @@ export function instanceWatch(
       ? createPathGetter(publicThis, source)
       : () => publicThis[source]
     : source.bind(publicThis)
+  let cb
+  if (isFunction(value)) {
+    cb = value
+  } else {
+    cb = value.handler as Function
+    options = value
+  }
   return doWatch(getter, cb.bind(publicThis), options, this)
 }
 
@@ -367,9 +400,9 @@ function traverse(value: unknown, seen: Set<unknown> = new Set()) {
     value.forEach((v: any) => {
       traverse(v, seen)
     })
-  } else {
+  } else if (isPlainObject(value)) {
     for (const key in value) {
-      traverse(value[key], seen)
+      traverse((value as any)[key], seen)
     }
   }
   return value
diff --git a/packages/runtime-core/src/compat/__tests__/global.spec.ts b/packages/runtime-core/src/compat/__tests__/global.spec.ts
new file mode 100644
index 00000000..cf1bb514
--- /dev/null
+++ b/packages/runtime-core/src/compat/__tests__/global.spec.ts
@@ -0,0 +1,336 @@
+import Vue from '@vue/compat'
+import { effect, isReactive } from '@vue/reactivity'
+import {
+  DeprecationTypes,
+  deprecationData,
+  toggleDeprecationWarning
+} from '../compatConfig'
+
+beforeEach(() => {
+  toggleDeprecationWarning(false)
+  Vue.configureCompat({ MODE: 2 })
+})
+
+afterEach(() => {
+  Vue.configureCompat({ MODE: 3 })
+  toggleDeprecationWarning(false)
+})
+
+describe('GLOBAL_MOUNT', () => {
+  test('new Vue() with el', () => {
+    toggleDeprecationWarning(true)
+
+    const el = document.createElement('div')
+    el.innerHTML = `{{ msg }}`
+    new Vue({
+      el,
+      compatConfig: { GLOBAL_MOUNT: true },
+      data() {
+        return {
+          msg: 'hello'
+        }
+      }
+    })
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_MOUNT].message
+    ).toHaveBeenWarned()
+    expect(el.innerHTML).toBe('hello')
+  })
+
+  test('new Vue() + $mount', () => {
+    const el = document.createElement('div')
+    el.innerHTML = `{{ msg }}`
+    new Vue({
+      data() {
+        return {
+          msg: 'hello'
+        }
+      }
+    }).$mount(el)
+    expect(el.innerHTML).toBe('hello')
+  })
+})
+
+describe('GLOBAL_MOUNT_CONTAINER', () => {
+  test('should warn', () => {
+    toggleDeprecationWarning(true)
+
+    const el = document.createElement('div')
+    el.innerHTML = `test`
+    el.setAttribute('v-bind:id', 'foo')
+    new Vue().$mount(el)
+    // warning only
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_MOUNT].message
+    ).toHaveBeenWarned()
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_MOUNT_CONTAINER].message
+    ).toHaveBeenWarned()
+  })
+})
+
+describe('GLOBAL_EXTEND', () => {
+  // https://github.com/vuejs/vue/blob/dev/test/unit/features/global-api/extend.spec.js
+  it('should correctly merge options', () => {
+    toggleDeprecationWarning(true)
+
+    const Test = Vue.extend({
+      name: 'test',
+      a: 1,
+      b: 2
+    })
+    expect(Test.options.a).toBe(1)
+    expect(Test.options.b).toBe(2)
+    expect(Test.super).toBe(Vue)
+    const t = new Test({
+      a: 2
+    })
+    expect(t.$options.a).toBe(2)
+    expect(t.$options.b).toBe(2)
+    // inheritance
+    const Test2 = Test.extend({
+      a: 2
+    })
+    expect(Test2.options.a).toBe(2)
+    expect(Test2.options.b).toBe(2)
+    const t2 = new Test2({
+      a: 3
+    })
+    expect(t2.$options.a).toBe(3)
+    expect(t2.$options.b).toBe(2)
+
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_MOUNT].message
+    ).toHaveBeenWarned()
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_EXTEND].message
+    ).toHaveBeenWarned()
+  })
+
+  it('should work when used as components', () => {
+    const foo = Vue.extend({
+      template: '<span>foo</span>'
+    })
+    const bar = Vue.extend({
+      template: '<span>bar</span>'
+    })
+    const vm = new Vue({
+      template: '<div><foo></foo><bar></bar></div>',
+      components: { foo, bar }
+    }).$mount()
+    expect(vm.$el.innerHTML).toBe('<span>foo</span><span>bar</span>')
+  })
+
+  it('should merge lifecycle hooks', () => {
+    const calls: number[] = []
+    const A = Vue.extend({
+      created() {
+        calls.push(1)
+      }
+    })
+    const B = A.extend({
+      created() {
+        calls.push(2)
+      }
+    })
+    new B({
+      created() {
+        calls.push(3)
+      }
+    })
+    expect(calls).toEqual([1, 2, 3])
+  })
+
+  it('should not merge nested mixins created with Vue.extend', () => {
+    const A = Vue.extend({
+      created: () => {}
+    })
+    const B = Vue.extend({
+      mixins: [A],
+      created: () => {}
+    })
+    const C = Vue.extend({
+      extends: B,
+      created: () => {}
+    })
+    const D = Vue.extend({
+      mixins: [C],
+      created: () => {}
+    })
+    expect(D.options.created!.length).toBe(4)
+  })
+
+  it('should merge methods', () => {
+    const A = Vue.extend({
+      methods: {
+        a() {
+          return this.n
+        }
+      }
+    })
+    const B = A.extend({
+      methods: {
+        b() {
+          return this.n + 1
+        }
+      }
+    })
+    const b = new B({
+      data: () => ({ n: 0 }),
+      methods: {
+        c() {
+          return this.n + 2
+        }
+      }
+    }) as any
+    expect(b.a()).toBe(0)
+    expect(b.b()).toBe(1)
+    expect(b.c()).toBe(2)
+  })
+
+  it('should merge assets', () => {
+    const A = Vue.extend({
+      components: {
+        aa: {
+          template: '<div>A</div>'
+        }
+      }
+    })
+    const B = A.extend({
+      components: {
+        bb: {
+          template: '<div>B</div>'
+        }
+      }
+    })
+    const b = new B({
+      template: '<div><aa></aa><bb></bb></div>'
+    }).$mount()
+    expect(b.$el.innerHTML).toBe('<div>A</div><div>B</div>')
+  })
+
+  it('caching', () => {
+    const options = {
+      template: '<div></div>'
+    }
+    const A = Vue.extend(options)
+    const B = Vue.extend(options)
+    expect(A).toBe(B)
+  })
+
+  it('extended options should use different identify from parent', () => {
+    const A = Vue.extend({ computed: {} })
+    const B = A.extend()
+    B.options.computed.b = () => 'foo'
+    expect(B.options.computed).not.toBe(A.options.computed)
+    expect(A.options.computed.b).toBeUndefined()
+  })
+})
+
+describe('GLOBAL_PROTOTYPE', () => {
+  test('plain properties', () => {
+    toggleDeprecationWarning(true)
+    Vue.prototype.$test = 1
+    const vm = new Vue() as any
+    expect(vm.$test).toBe(1)
+    delete Vue.prototype.$test
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_MOUNT].message
+    ).toHaveBeenWarned()
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_PROTOTYPE].message
+    ).toHaveBeenWarned()
+  })
+
+  test('method this context', () => {
+    Vue.prototype.$test = function() {
+      return this.msg
+    }
+    const vm = new Vue({
+      data() {
+        return { msg: 'method' }
+      }
+    }) as any
+    expect(vm.$test()).toBe('method')
+    delete Vue.prototype.$test
+  })
+
+  test('defined properties', () => {
+    Object.defineProperty(Vue.prototype, '$test', {
+      configurable: true,
+      get() {
+        return this.msg
+      }
+    })
+    const vm = new Vue({
+      data() {
+        return { msg: 'getter' }
+      }
+    }) as any
+    expect(vm.$test).toBe('getter')
+    delete Vue.prototype.$test
+  })
+
+  test('extended prototype', async () => {
+    const Foo = Vue.extend()
+    Foo.prototype.$test = 1
+    const vm = new Foo() as any
+    expect(vm.$test).toBe(1)
+    const plain = new Vue() as any
+    expect(plain.$test).toBeUndefined()
+  })
+})
+
+describe('GLOBAL_SET/DELETE', () => {
+  test('set', () => {
+    toggleDeprecationWarning(true)
+    const obj: any = {}
+    Vue.set(obj, 'foo', 1)
+    expect(obj.foo).toBe(1)
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_SET].message
+    ).toHaveBeenWarned()
+  })
+
+  test('delete', () => {
+    toggleDeprecationWarning(true)
+    const obj: any = { foo: 1 }
+    Vue.delete(obj, 'foo')
+    expect('foo' in obj).toBe(false)
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_DELETE].message
+    ).toHaveBeenWarned()
+  })
+})
+
+describe('GLOBAL_OBSERVABLE', () => {
+  test('should work', () => {
+    toggleDeprecationWarning(true)
+    const obj = Vue.observable({})
+    expect(isReactive(obj)).toBe(true)
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_OBSERVABLE].message
+    ).toHaveBeenWarned()
+  })
+})
+
+describe('GLOBAL_PRIVATE_UTIL', () => {
+  test('defineReactive', () => {
+    toggleDeprecationWarning(true)
+    const obj: any = {}
+    // @ts-ignore
+    Vue.util.defineReactive(obj, 'test', 1)
+
+    let n
+    effect(() => {
+      n = obj.test
+    })
+    expect(n).toBe(1)
+    obj.test++
+    expect(n).toBe(2)
+
+    expect(
+      deprecationData[DeprecationTypes.GLOBAL_PRIVATE_UTIL].message
+    ).toHaveBeenWarned()
+  })
+})
diff --git a/packages/runtime-core/src/compat/__tests__/globalConfig.spec.ts b/packages/runtime-core/src/compat/__tests__/globalConfig.spec.ts
new file mode 100644
index 00000000..d7a7bc26
--- /dev/null
+++ b/packages/runtime-core/src/compat/__tests__/globalConfig.spec.ts
@@ -0,0 +1,77 @@
+import Vue from '@vue/compat'
+import { toggleDeprecationWarning } from '../compatConfig'
+
+beforeEach(() => {
+  toggleDeprecationWarning(false)
+  Vue.configureCompat({ MODE: 2 })
+})
+
+afterEach(() => {
+  Vue.configureCompat({ MODE: 3 })
+  toggleDeprecationWarning(false)
+})
+
+function triggerEvent(
+  target: Element,
+  event: string,
+  process?: (e: any) => any
+) {
+  const e = document.createEvent('HTMLEvents')
+  e.initEvent(event, true, true)
+  if (process) process(e)
+  target.dispatchEvent(e)
+  return e
+}
+
+// only testing config options that affect runtime behavior.
+
+test('GLOBAL_KEY_CODES', () => {
+  Vue.config.keyCodes = {
+    foo: 86,
+    bar: [38, 87]
+  }
+
+  const onFoo = jest.fn()
+  const onBar = jest.fn()
+
+  const el = document.createElement('div')
+  new Vue({
+    el,
+    template: `<input type="text" @keyup.foo="onFoo" @keyup.bar="onBar">`,
+    methods: {
+      onFoo,
+      onBar
+    }
+  })
+
+  triggerEvent(el.children[0], 'keyup', e => {
+    e.key = '_'
+    e.keyCode = 86
+  })
+  expect(onFoo).toHaveBeenCalledTimes(1)
+  expect(onBar).toHaveBeenCalledTimes(0)
+
+  triggerEvent(el.children[0], 'keyup', e => {
+    e.key = '_'
+    e.keyCode = 38
+  })
+  expect(onFoo).toHaveBeenCalledTimes(1)
+  expect(onBar).toHaveBeenCalledTimes(1)
+
+  triggerEvent(el.children[0], 'keyup', e => {
+    e.key = '_'
+    e.keyCode = 87
+  })
+  expect(onFoo).toHaveBeenCalledTimes(1)
+  expect(onBar).toHaveBeenCalledTimes(2)
+})
+
+test('GLOBAL_IGNORED_ELEMENTS', () => {
+  Vue.config.ignoredElements = [/^v-/, 'foo']
+  const el = document.createElement('div')
+  new Vue({
+    el,
+    template: `<v-foo/><foo/>`
+  })
+  expect(el.innerHTML).toBe(`<v-foo></v-foo><foo></foo>`)
+})
diff --git a/packages/runtime-core/src/compat/__tests__/renderFn.spec.ts b/packages/runtime-core/src/compat/__tests__/renderFn.spec.ts
new file mode 100644
index 00000000..7c2059d7
--- /dev/null
+++ b/packages/runtime-core/src/compat/__tests__/renderFn.spec.ts
@@ -0,0 +1,178 @@
+import { ShapeFlags } from '@vue/shared/src'
+import { createComponentInstance } from '../../component'
+import { setCurrentRenderingInstance } from '../../componentRenderContext'
+import { DirectiveBinding } from '../../directives'
+import { createVNode } from '../../vnode'
+import { compatH as h } from '../renderFn'
+
+describe('compat: render function', () => {
+  const mockDir = {}
+  const mockChildComp = {}
+  const mockComponent = {
+    directives: {
+      mockDir
+    },
+    components: {
+      foo: mockChildComp
+    }
+  }
+  const mockInstance = createComponentInstance(
+    createVNode(mockComponent),
+    null,
+    null
+  )
+  beforeEach(() => {
+    setCurrentRenderingInstance(mockInstance)
+  })
+  afterEach(() => {
+    setCurrentRenderingInstance(null)
+  })
+
+  test('string component lookup', () => {
+    expect(h('foo')).toMatchObject({
+      type: mockChildComp
+    })
+  })
+
+  test('class / style / attrs / domProps / props', () => {
+    expect(
+      h('div', {
+        class: 'foo',
+        style: { color: 'red' },
+        attrs: {
+          id: 'foo'
+        },
+        domProps: {
+          innerHTML: 'hi'
+        },
+        props: {
+          myProp: 'foo'
+        }
+      })
+    ).toMatchObject({
+      props: {
+        class: 'foo',
+        style: { color: 'red' },
+        id: 'foo',
+        innerHTML: 'hi',
+        myProp: 'foo'
+      }
+    })
+  })
+
+  test('staticClass + class', () => {
+    expect(
+      h('div', {
+        class: { foo: true },
+        staticClass: 'bar'
+      })
+    ).toMatchObject({
+      props: {
+        class: 'bar foo'
+      }
+    })
+  })
+
+  test('staticStyle + style', () => {
+    expect(
+      h('div', {
+        style: { color: 'red' },
+        staticStyle: { fontSize: '14px' }
+      })
+    ).toMatchObject({
+      props: {
+        style: {
+          color: 'red',
+          fontSize: '14px'
+        }
+      }
+    })
+  })
+
+  test('on / nativeOn', () => {
+    const fn = () => {}
+    expect(
+      h('div', {
+        on: {
+          click: fn,
+          fooBar: fn
+        },
+        nativeOn: {
+          click: fn,
+          'bar-baz': fn
+        }
+      })
+    ).toMatchObject({
+      props: {
+        onClick: fn, // should dedupe
+        onFooBar: fn,
+        'onBar-baz': fn
+      }
+    })
+  })
+
+  test('directives', () => {
+    expect(
+      h('div', {
+        directives: [
+          {
+            name: 'mock-dir',
+            value: '2',
+            // expression: '1 + 1',
+            arg: 'foo',
+            modifiers: {
+              bar: true
+            }
+          }
+        ]
+      })
+    ).toMatchObject({
+      dirs: [
+        {
+          dir: mockDir,
+          instance: mockInstance.proxy,
+          value: '2',
+          oldValue: void 0,
+          arg: 'foo',
+          modifiers: {
+            bar: true
+          }
+        }
+      ] as DirectiveBinding[]
+    })
+  })
+
+  test('scopedSlots', () => {
+    const scopedSlots = {
+      default() {}
+    }
+    const vnode = h(mockComponent, {
+      scopedSlots
+    })
+    expect(vnode).toMatchObject({
+      children: scopedSlots
+    })
+    expect('scopedSlots' in vnode.props!).toBe(false)
+    expect(vnode.shapeFlag & ShapeFlags.SLOTS_CHILDREN).toBeTruthy()
+  })
+
+  test('legacy named slot', () => {
+    const vnode = h(mockComponent, [
+      'text',
+      h('div', { slot: 'foo' }, 'one'),
+      h('div', { slot: 'bar' }, 'two'),
+      h('div', { slot: 'foo' }, 'three'),
+      h('div', 'four')
+    ])
+    expect(vnode.shapeFlag & ShapeFlags.SLOTS_CHILDREN).toBeTruthy()
+    const slots = vnode.children as any
+
+    // default
+    expect(slots.default()).toMatchObject(['text', { children: 'four' }])
+    expect(slots.foo()).toMatchObject([
+      { children: 'one' },
+      { children: 'three' }
+    ])
+    expect(slots.bar()).toMatchObject([{ children: 'two' }])
+  })
+})
diff --git a/packages/runtime-core/src/compat/attrsFallthrough.ts b/packages/runtime-core/src/compat/attrsFallthrough.ts
new file mode 100644
index 00000000..a23890b6
--- /dev/null
+++ b/packages/runtime-core/src/compat/attrsFallthrough.ts
@@ -0,0 +1,27 @@
+import { isOn } from '@vue/shared'
+import { ComponentInternalInstance } from '../component'
+import { DeprecationTypes, isCompatEnabled } from './compatConfig'
+
+export function shouldSkipAttr(
+  key: string,
+  value: any,
+  instance: ComponentInternalInstance
+): boolean {
+  if (
+    (key === 'class' || key === 'style') &&
+    isCompatEnabled(DeprecationTypes.INSTANCE_ATTRS_CLASS_STYLE, instance)
+  ) {
+    return true
+  }
+  if (
+    isOn(key) &&
+    isCompatEnabled(DeprecationTypes.INSTANCE_LISTENERS, instance)
+  ) {
+    return true
+  }
+  // vue-router
+  if (key.startsWith('routerView') || key === 'registerRouteInstance') {
+    return true
+  }
+  return false
+}
diff --git a/packages/runtime-core/src/compat/compatConfig.ts b/packages/runtime-core/src/compat/compatConfig.ts
new file mode 100644
index 00000000..e59bdb0a
--- /dev/null
+++ b/packages/runtime-core/src/compat/compatConfig.ts
@@ -0,0 +1,636 @@
+import { extend, hasOwn, isArray } from '@vue/shared'
+import {
+  ComponentInternalInstance,
+  ComponentOptions,
+  formatComponentName,
+  getComponentName,
+  getCurrentInstance,
+  isRuntimeOnly
+} from '../component'
+import { warn } from '../warning'
+
+export const enum DeprecationTypes {
+  GLOBAL_MOUNT = 'GLOBAL_MOUNT',
+  GLOBAL_MOUNT_CONTAINER = 'GLOBAL_MOUNT_CONTAINER',
+  GLOBAL_EXTEND = 'GLOBAL_EXTEND',
+  GLOBAL_PROTOTYPE = 'GLOBAL_PROTOTYPE',
+  GLOBAL_SET = 'GLOBAL_SET',
+  GLOBAL_DELETE = 'GLOBAL_DELETE',
+  GLOBAL_OBSERVABLE = 'GLOBAL_OBSERVABLE',
+  GLOBAL_PRIVATE_UTIL = 'GLOBAL_PRIVATE_UTIL',
+
+  CONFIG_SILENT = 'CONFIG_SILENT',
+  CONFIG_DEVTOOLS = 'CONFIG_DEVTOOLS',
+  CONFIG_KEY_CODES = 'CONFIG_KEY_CODES',
+  CONFIG_PRODUCTION_TIP = 'CONFIG_PRODUCTION_TIP',
+  CONFIG_IGNORED_ELEMENTS = 'CONFIG_IGNORED_ELEMENTS',
+  CONFIG_WHITESPACE = 'CONFIG_WHITESPACE',
+
+  INSTANCE_SET = 'INSTANCE_SET',
+  INSTANCE_DELETE = 'INSTANCE_DELETE',
+  INSTANCE_DESTROY = 'INSTANCE_DESTROY',
+  INSTANCE_EVENT_EMITTER = 'INSTANCE_EVENT_EMITTER',
+  INSTANCE_EVENT_HOOKS = 'INSTANCE_EVENT_HOOKS',
+  INSTANCE_CHILDREN = 'INSTANCE_CHILDREN',
+  INSTANCE_LISTENERS = 'INSTANCE_LISTENERS',
+  INSTANCE_SCOPED_SLOTS = 'INSTANCE_SCOPED_SLOTS',
+  INSTANCE_ATTRS_CLASS_STYLE = 'INSTANCE_ATTRS_CLASS_STYLE',
+
+  OPTIONS_DATA_FN = 'OPTIONS_DATA_FN',
+  OPTIONS_DATA_MERGE = 'OPTIONS_DATA_MERGE',
+  OPTIONS_BEFORE_DESTROY = 'OPTIONS_BEFORE_DESTROY',
+  OPTIONS_DESTROYED = 'OPTIONS_DESTROYED',
+
+  WATCH_ARRAY = 'WATCH_ARRAY',
+  PROPS_DEFAULT_THIS = 'PROPS_DEFAULT_THIS',
+
+  V_FOR_REF = 'V_FOR_REF',
+  V_ON_KEYCODE_MODIFIER = 'V_ON_KEYCODE_MODIFIER',
+  CUSTOM_DIR = 'CUSTOM_DIR',
+
+  ATTR_FALSE_VALUE = 'ATTR_FALSE_VALUE',
+  ATTR_ENUMERATED_COERSION = 'ATTR_ENUMERATED_COERSION',
+
+  TRANSITION_CLASSES = 'TRANSITION_CLASSES',
+  TRANSITION_GROUP_ROOT = 'TRANSITION_GROUP_ROOT',
+
+  COMPONENT_ASYNC = 'COMPONENT_ASYNC',
+  COMPONENT_FUNCTIONAL = 'COMPONENT_FUNCTIONAL',
+  COMPONENT_V_MODEL = 'COMPONENT_V_MODEL',
+
+  RENDER_FUNCTION = 'RENDER_FUNCTION',
+
+  FILTERS = 'FILTERS',
+
+  PRIVATE_APIS = 'PRIVATE_APIS'
+}
+
+type DeprecationData = {
+  message: string | ((...args: any[]) => string)
+  link?: string
+}
+
+export const deprecationData: Record<DeprecationTypes, DeprecationData> = {
+  [DeprecationTypes.GLOBAL_MOUNT]: {
+    message:
+      `The global app bootstrapping API has changed: vm.$mount() and the "el" ` +
+      `option have been removed. Use createApp(RootComponent).mount() instead.`,
+    link: `https://v3.vuejs.org/guide/migration/global-api.html#mounting-app-instance`
+  },
+
+  [DeprecationTypes.GLOBAL_MOUNT_CONTAINER]: {
+    message:
+      `Vue detected directives on the mount container. ` +
+      `In Vue 3, the container is no longer considered part of the template ` +
+      `and will not be processed/replaced.`,
+    link: `https://v3.vuejs.org/guide/migration/mount-changes.html`
+  },
+
+  [DeprecationTypes.GLOBAL_EXTEND]: {
+    message:
+      `Vue.extend() has been removed in Vue 3. ` +
+      `Use defineComponent() instead.`,
+    link: `https://v3.vuejs.org/api/global-api.html#definecomponent`
+  },
+
+  [DeprecationTypes.GLOBAL_PROTOTYPE]: {
+    message:
+      `Vue.prototype is no longer available in Vue 3. ` +
+      `Use config.globalProperties instead.`,
+    link: `https://v3.vuejs.org/guide/migration/global-api.html#vue-prototype-replaced-by-config-globalproperties`
+  },
+
+  [DeprecationTypes.GLOBAL_SET]: {
+    message:
+      `Vue.set() has been removed as it is no longer needed in Vue 3. ` +
+      `Simply use native JavaScript mutations.`
+  },
+
+  [DeprecationTypes.GLOBAL_DELETE]: {
+    message:
+      `Vue.delete() has been removed as it is no longer needed in Vue 3. ` +
+      `Simply use native JavaScript mutations.`
+  },
+
+  [DeprecationTypes.GLOBAL_OBSERVABLE]: {
+    message:
+      `Vue.observable() has been removed. ` +
+      `Use \`import { reactive } from "vue"\` from Composition API instead.`,
+    link: `https://v3.vuejs.org/api/basic-reactivity.html`
+  },
+
+  [DeprecationTypes.GLOBAL_PRIVATE_UTIL]: {
+    message:
+      `Vue.util has been removed. Please refactor to avoid its usage ` +
+      `since it was an internal API even in Vue 2.`
+  },
+
+  [DeprecationTypes.CONFIG_SILENT]: {
+    message:
+      `config.silent has been removed because it is not good practice to ` +
+      `intentionally suppress warnings. You can use your browser console's ` +
+      `filter features to focus on relevant messages.`
+  },
+
+  [DeprecationTypes.CONFIG_DEVTOOLS]: {
+    message:
+      `config.devtools has been removed. To enable devtools for ` +
+      `production, configure the __VUE_PROD_DEVTOOLS__ compile-time flag.`,
+    link: `https://github.com/vuejs/vue-next/tree/master/packages/vue#bundler-build-feature-flags`
+  },
+
+  [DeprecationTypes.CONFIG_KEY_CODES]: {
+    message:
+      `config.keyCodes has been removed. ` +
+      `In Vue 3, you can directly use the kebab-case key names as v-on modifiers.`,
+    link: `https://v3.vuejs.org/guide/migration/keycode-modifiers.html`
+  },
+
+  [DeprecationTypes.CONFIG_PRODUCTION_TIP]: {
+    message: `config.productionTip has been removed.`,
+    link: `https://v3.vuejs.org/guide/migration/global-api.html#config-productiontip-removed`
+  },
+
+  [DeprecationTypes.CONFIG_IGNORED_ELEMENTS]: {
+    message: () => {
+      let msg = `config.ignoredElements has been removed.`
+      if (isRuntimeOnly()) {
+        msg += ` Pass the "isCustomElement" option to @vue/compiler-dom instead.`
+      } else {
+        msg += ` Use config.isCustomElement instead.`
+      }
+      return msg
+    },
+    link: `https://v3.vuejs.org/guide/migration/global-api.html#config-ignoredelements-is-now-config-iscustomelement`
+  },
+
+  [DeprecationTypes.CONFIG_WHITESPACE]: {
+    // this warning is only relevant in the full build when using runtime
+    // compilation, so it's put in the runtime compatConfig list.
+    message:
+      `Vue 3 compiler's whitespace option will default to "condense" instead of ` +
+      `"preserve". To suppress this warning, provide an explicit value for ` +
+      `\`config.compilerOptions.whitespace\`.`
+  },
+
+  [DeprecationTypes.INSTANCE_SET]: {
+    message:
+      `vm.$set() has been removed as it is no longer needed in Vue 3. ` +
+      `Simply use native JavaScript mutations.`
+  },
+
+  [DeprecationTypes.INSTANCE_DELETE]: {
+    message:
+      `vm.$delete() has been removed as it is no longer needed in Vue 3. ` +
+      `Simply use native JavaScript mutations.`
+  },
+
+  [DeprecationTypes.INSTANCE_DESTROY]: {
+    message: `vm.$destroy() has been removed. Use app.unmount() instead.`,
+    link: `https://v3.vuejs.org/api/application-api.html#unmount`
+  },
+
+  [DeprecationTypes.INSTANCE_EVENT_EMITTER]: {
+    message:
+      `vm.$on/$once/$off() have been removed. ` +
+      `Use an external event emitter library instead.`,
+    link: `https://v3.vuejs.org/guide/migration/events-api.html`
+  },
+
+  [DeprecationTypes.INSTANCE_EVENT_HOOKS]: {
+    message: event =>
+      `"${event}" lifecycle events are no longer supported. From templates, ` +
+      `use the "vnode" prefix instead of "hook:". For example, @${event} ` +
+      `should be changed to @vnode-${event.slice(5)}. ` +
+      `From JavaScript, use Composition API to dynamically register lifecycle ` +
+      `hooks.`,
+    link: `https://v3.vuejs.org/guide/migration/vnode-lifecycle-events.html`
+  },
+
+  [DeprecationTypes.INSTANCE_CHILDREN]: {
+    message:
+      `vm.$children has been removed. Consider refactoring your logic ` +
+      `to avoid relying on direct access to child components.`,
+    link: `https://v3.vuejs.org/guide/migration/children.html`
+  },
+
+  [DeprecationTypes.INSTANCE_LISTENERS]: {
+    message:
+      `vm.$listeners has been removed. In Vue 3, parent v-on listeners are ` +
+      `included in vm.$attrs and it is no longer necessary to separately use ` +
+      `v-on="$listeners" if you are already using v-bind="$attrs". ` +
+      `(Note: the Vue 3 behavior only applies if this compat config is disabled)`,
+    link: `https://v3.vuejs.org/guide/migration/listeners-removed.html`
+  },
+
+  [DeprecationTypes.INSTANCE_SCOPED_SLOTS]: {
+    message: `vm.$scopedSlots has been removed. Use vm.$slots instead.`,
+    link: `https://v3.vuejs.org/guide/migration/slots-unification.html`
+  },
+
+  [DeprecationTypes.INSTANCE_ATTRS_CLASS_STYLE]: {
+    message: componentName =>
+      `Component <${componentName}> has \`inheritAttrs: false\` but is ` +
+      `relying on class/style fallthrough from parent. In Vue 3, class/style ` +
+      `are now included in $attrs and will no longer fallthrough when ` +
+      `inheritAttrs is false. If you are already using v-bind="$attrs" on ` +
+      `component root it should render the same end result. ` +
+      `If you are binding $attrs to a non-root element and expecting ` +
+      `class/style to fallthrough on root, you will need to now manually bind ` +
+      `them on root via :class="$attrs.class".`,
+    link: `https://v3.vuejs.org/guide/migration/attrs-includes-class-style.html`
+  },
+
+  [DeprecationTypes.OPTIONS_DATA_FN]: {
+    message:
+      `The "data" option can no longer be a plain object. ` +
+      `Always use a function.`,
+    link: `https://v3.vuejs.org/guide/migration/data-option.html`
+  },
+
+  [DeprecationTypes.OPTIONS_DATA_MERGE]: {
+    message: (key: string) =>
+      `Detected conflicting key "${key}" when merging data option values. ` +
+      `In Vue 3, data keys are merged shallowly and will override one another.`,
+    link: `https://v3.vuejs.org/guide/migration/data-option.html#mixin-merge-behavior-change`
+  },
+
+  [DeprecationTypes.OPTIONS_BEFORE_DESTROY]: {
+    message: `\`beforeDestroy\` has been renamed to \`beforeUnmount\`.`
+  },
+
+  [DeprecationTypes.OPTIONS_DESTROYED]: {
+    message: `\`destroyed\` has been renamed to \`unmounted\`.`
+  },
+
+  [DeprecationTypes.WATCH_ARRAY]: {
+    message:
+      `"watch" option or vm.$watch on an array value will no longer ` +
+      `trigger on array mutation unless the "deep" option is specified. ` +
+      `If current usage is intended, you can disable the compat behavior and ` +
+      `suppress this warning with:` +
+      `\n\n  configureCompat({ ${DeprecationTypes.WATCH_ARRAY}: false })\n`,
+    link: `https://v3.vuejs.org/guide/migration/watch.html`
+  },
+
+  [DeprecationTypes.PROPS_DEFAULT_THIS]: {
+    message: (key: string) =>
+      `props default value function no longer has access to "this". The compat ` +
+      `build only offers access to this.$options.` +
+      `(found in prop "${key}")`,
+    link: `https://v3.vuejs.org/guide/migration/props-default-this.html`
+  },
+
+  [DeprecationTypes.CUSTOM_DIR]: {
+    message: (legacyHook: string, newHook: string) =>
+      `Custom directive hook "${legacyHook}" has been removed. ` +
+      `Use "${newHook}" instead.`,
+    link: `https://v3.vuejs.org/guide/migration/custom-directives.html`
+  },
+
+  [DeprecationTypes.V_FOR_REF]: {
+    message:
+      `Ref usage on v-for no longer creates array ref values in Vue 3. ` +
+      `Consider using function refs or refactor to avoid ref usage altogether.`,
+    link: `https://v3.vuejs.org/guide/migration/array-refs.html`
+  },
+
+  [DeprecationTypes.V_ON_KEYCODE_MODIFIER]: {
+    message:
+      `Using keyCode as v-on modifier is no longer supported. ` +
+      `Use kebab-case key name modifiers instead.`,
+    link: `https://v3.vuejs.org/guide/migration/keycode-modifiers.html`
+  },
+
+  [DeprecationTypes.ATTR_FALSE_VALUE]: {
+    message: (name: string) =>
+      `Attribute "${name}" with v-bind value \`false\` will render ` +
+      `${name}="false" instead of removing it in Vue 3. To remove the attribute, ` +
+      `use \`null\` or \`undefined\` instead. If the usage is intended, ` +
+      `you can disable the compat behavior and suppress this warning with:` +
+      `\n\n  configureCompat({ ${
+        DeprecationTypes.ATTR_FALSE_VALUE
+      }: false })\n`,
+    link: `https://v3.vuejs.org/guide/migration/attribute-coercion.html`
+  },
+
+  [DeprecationTypes.ATTR_ENUMERATED_COERSION]: {
+    message: (name: string, value: any, coerced: string) =>
+      `Enumerated attribute "${name}" with v-bind value \`${value}\` will ` +
+      `${
+        value === null ? `be removed` : `render the value as-is`
+      } instead of coercing the value to "${coerced}" in Vue 3. ` +
+      `Always use explicit "true" or "false" values for enumerated attributes. ` +
+      `If the usage is intended, ` +
+      `you can disable the compat behavior and suppress this warning with:` +
+      `\n\n  configureCompat({ ${
+        DeprecationTypes.ATTR_ENUMERATED_COERSION
+      }: false })\n`,
+    link: `https://v3.vuejs.org/guide/migration/attribute-coercion.html`
+  },
+
+  [DeprecationTypes.TRANSITION_CLASSES]: {
+    message: `` // this feature cannot be runtime-detected
+  },
+
+  [DeprecationTypes.TRANSITION_GROUP_ROOT]: {
+    message:
+      `<TransitionGroup> no longer renders a root <span> element by ` +
+      `default if no "tag" prop is specified. If you do not rely on the span ` +
+      `for styling, you can disable the compat behavior and suppress this ` +
+      `warning with:` +
+      `\n\n  configureCompat({ ${
+        DeprecationTypes.TRANSITION_GROUP_ROOT
+      }: false })\n`,
+    link: `https://v3.vuejs.org/guide/migration/transition-group.html`
+  },
+
+  [DeprecationTypes.COMPONENT_ASYNC]: {
+    message: (comp: any) => {
+      const name = getComponentName(comp)
+      return (
+        `Async component${
+          name ? ` <${name}>` : `s`
+        } should be explicitly created via \`defineAsyncComponent()\` ` +
+        `in Vue 3. Plain functions will be treated as functional components in ` +
+        `non-compat build. If you have already migrated all async component ` +
+        `usage and intend to use plain functions for functional components, ` +
+        `you can disable the compat behavior and suppress this ` +
+        `warning with:` +
+        `\n\n  configureCompat({ ${
+          DeprecationTypes.COMPONENT_ASYNC
+        }: false })\n`
+      )
+    },
+    link: `https://v3.vuejs.org/guide/migration/async-components.html`
+  },
+
+  [DeprecationTypes.COMPONENT_FUNCTIONAL]: {
+    message: (comp: any) => {
+      const name = getComponentName(comp)
+      return (
+        `Functional component${
+          name ? ` <${name}>` : `s`
+        } should be defined as a plain function in Vue 3. The "functional" ` +
+        `option has been removed. NOTE: Before migrating to use plain ` +
+        `functions for functional components, first make sure that all async ` +
+        `components usage have been migrated and its compat behavior has ` +
+        `been disabled.`
+      )
+    },
+    link: `https://v3.vuejs.org/guide/migration/functional-components.html`
+  },
+
+  [DeprecationTypes.COMPONENT_V_MODEL]: {
+    message: (comp: ComponentOptions) => {
+      const configMsg =
+        `opt-in to ` +
+        `Vue 3 behavior on a per-component basis with \`compatConfig: { ${
+          DeprecationTypes.COMPONENT_V_MODEL
+        }: false }\`.`
+      if (
+        comp.props && isArray(comp.props)
+          ? comp.props.includes('modelValue')
+          : hasOwn(comp.props, 'modelValue')
+      ) {
+        return (
+          `Component delcares "modelValue" prop, which is Vue 3 usage, but ` +
+          `is running under Vue 2 compat v-model behavior. You can ${configMsg}`
+        )
+      }
+      return (
+        `v-model usage on component has changed in Vue 3. Component that expects ` +
+        `to work with v-model should now use the "modelValue" prop and emit the ` +
+        `"update:modelValue" event. You can update the usage and then ${configMsg}`
+      )
+    },
+    link: `https://v3.vuejs.org/guide/migration/v-model.html`
+  },
+
+  [DeprecationTypes.RENDER_FUNCTION]: {
+    message:
+      `Vue 3's render function API has changed. ` +
+      `You can opt-in to the new API with:` +
+      `\n\n  configureCompat({ ${
+        DeprecationTypes.RENDER_FUNCTION
+      }: false })\n` +
+      `\n  (This can also be done per-component via the "compatConfig" option.)`,
+    link: `https://v3.vuejs.org/guide/migration/render-function-api.html`
+  },
+
+  [DeprecationTypes.FILTERS]: {
+    message:
+      `filters have been removed in Vue 3. ` +
+      `The "|" symbol will be treated as native JavaScript bitwise OR operator. ` +
+      `Use method calls or computed properties instead.`,
+    link: `https://v3.vuejs.org/guide/migration/filters.html`
+  },
+
+  [DeprecationTypes.PRIVATE_APIS]: {
+    message: name =>
+      `"${name}" is a Vue 2 private API that no longer exists in Vue 3. ` +
+      `If you are seeing this warning only due to a dependency, you can ` +
+      `suppress this warning via { PRIVATE_APIS: 'supress-warning' }.`
+  }
+}
+
+const instanceWarned: Record<string, true> = Object.create(null)
+const warnCount: Record<string, number> = Object.create(null)
+
+// test only
+let warningEnabled = true
+
+export function toggleDeprecationWarning(flag: boolean) {
+  warningEnabled = flag
+}
+
+export function warnDeprecation(
+  key: DeprecationTypes,
+  instance: ComponentInternalInstance | null,
+  ...args: any[]
+) {
+  if (!__DEV__) {
+    return
+  }
+  if (__TEST__ && !warningEnabled) {
+    return
+  }
+
+  instance = instance || getCurrentInstance()
+
+  // check user config
+  const config = getCompatConfigForKey(key, instance)
+  if (config === 'suppress-warning') {
+    return
+  }
+
+  const dupKey = key + args.join('')
+  let compId: string | number | null =
+    instance && formatComponentName(instance, instance.type)
+  if (compId === 'Anonymous' && instance) {
+    compId = instance.uid
+  }
+
+  // skip if the same warning is emitted for the same component type
+  const componentDupKey = dupKey + compId
+  if (!__TEST__ && componentDupKey in instanceWarned) {
+    return
+  }
+  instanceWarned[componentDupKey] = true
+
+  // same warning, but different component. skip the long message and just
+  // log the key and count.
+  if (!__TEST__ && dupKey in warnCount) {
+    warn(`(deprecation ${key}) (${++warnCount[dupKey] + 1})`)
+    return
+  }
+
+  warnCount[dupKey] = 0
+
+  const { message, link } = deprecationData[key]
+  warn(
+    `(deprecation ${key}) ${
+      typeof message === 'function' ? message(...args) : message
+    }${link ? `\n  Details: ${link}` : ``}`
+  )
+  if (!isCompatEnabled(key, instance)) {
+    console.error(
+      `^ The above deprecation's compat behavior is disabled and will likely ` +
+        `lead to runtime errors.`
+    )
+  }
+}
+
+export type CompatConfig = Partial<
+  Record<DeprecationTypes, boolean | 'suppress-warning'>
+> & {
+  MODE?: 2 | 3
+}
+
+export const globalCompatConfig: CompatConfig = {
+  MODE: 2
+}
+
+export function configureCompat(config: CompatConfig) {
+  if (__DEV__) {
+    validateCompatConfig(config)
+  }
+  extend(globalCompatConfig, config)
+}
+
+const seenConfigObjects = /*#__PURE__*/ new WeakSet<CompatConfig>()
+const warnedInvalidKeys: Record<string, boolean> = {}
+
+// dev only
+export function validateCompatConfig(config: CompatConfig) {
+  if (seenConfigObjects.has(config)) {
+    return
+  }
+  seenConfigObjects.add(config)
+
+  for (const key of Object.keys(config)) {
+    if (
+      key !== 'MODE' &&
+      !(key in deprecationData) &&
+      !(key in warnedInvalidKeys)
+    ) {
+      if (key.startsWith('COMPILER_')) {
+        if (isRuntimeOnly()) {
+          warn(
+            `Depreaction config "${key}" is compiler-specific and you are ` +
+              `running a runtime-only build of Vue. This deprecation should be ` +
+              `configured via compiler options in your build setup instead.`
+            // TODO link to migration build docs on build setup
+          )
+        }
+      } else {
+        warn(`Invalid deprecation config "${key}".`)
+      }
+      warnedInvalidKeys[key] = true
+    }
+  }
+}
+
+export function getCompatConfigForKey(
+  key: DeprecationTypes | 'MODE',
+  instance: ComponentInternalInstance | null
+) {
+  const instanceConfig =
+    instance && (instance.type as ComponentOptions).compatConfig
+  if (instanceConfig && key in instanceConfig) {
+    return instanceConfig[key]
+  }
+  return globalCompatConfig[key]
+}
+
+export function isCompatEnabled(
+  key: DeprecationTypes,
+  instance: ComponentInternalInstance | null
+): boolean {
+  // skip compat for built-in components
+  if (instance && instance.type.__isBuiltIn) {
+    return false
+  }
+
+  const mode = getCompatConfigForKey('MODE', instance) || 2
+  const val = getCompatConfigForKey(key, instance)
+  if (mode === 2) {
+    return val !== false
+  } else {
+    return val === true || val === 'suppress-warning'
+  }
+}
+
+/**
+ * Use this for features that are completely removed in non-compat build.
+ */
+export function assertCompatEnabled(
+  key: DeprecationTypes,
+  instance: ComponentInternalInstance | null,
+  ...args: any[]
+) {
+  if (!isCompatEnabled(key, instance)) {
+    throw new Error(`${key} compat has been disabled.`)
+  } else if (__DEV__) {
+    warnDeprecation(key, instance, ...args)
+  }
+}
+
+/**
+ * Use this for features where legacy usage is still possible, but will likely
+ * lead to runtime error if compat is disabled. (warn in all cases)
+ */
+export function softAssertCompatEnabled(
+  key: DeprecationTypes,
+  instance: ComponentInternalInstance | null,
+  ...args: any[]
+) {
+  if (__DEV__) {
+    warnDeprecation(key, instance, ...args)
+  }
+  return isCompatEnabled(key, instance)
+}
+
+/**
+ * Use this for features with the same syntax but with mutually exclusive
+ * behavior in 2 vs 3. Only warn if compat is enabled.
+ * e.g. render function
+ */
+export function checkCompatEnabled(
+  key: DeprecationTypes,
+  instance: ComponentInternalInstance | null,
+  ...args: any[]
+) {
+  const enabled = isCompatEnabled(key, instance)
+  if (__DEV__ && enabled) {
+    warnDeprecation(key, instance, ...args)
+  }
+  return enabled
+}
+
+// run tests in v3 mode by default
+if (__TEST__) {
+  configureCompat({
+    MODE: 3
+  })
+}
diff --git a/packages/runtime-core/src/compat/component.ts b/packages/runtime-core/src/compat/component.ts
new file mode 100644
index 00000000..3e89924f
--- /dev/null
+++ b/packages/runtime-core/src/compat/component.ts
@@ -0,0 +1,164 @@
+import { isArray, isFunction, isObject, isPromise } from '@vue/shared'
+import { defineAsyncComponent } from '../apiAsyncComponent'
+import {
+  Component,
+  ComponentInternalInstance,
+  ComponentOptions,
+  FunctionalComponent,
+  getCurrentInstance
+} from '../component'
+import { resolveInjections } from '../componentOptions'
+import { InternalSlots } from '../componentSlots'
+import { isVNode } from '../vnode'
+import {
+  checkCompatEnabled,
+  softAssertCompatEnabled,
+  DeprecationTypes
+} from './compatConfig'
+import { getCompatListeners } from './instanceListeners'
+import { compatH } from './renderFn'
+
+export function convertLegacyComponent(
+  comp: any,
+  instance: ComponentInternalInstance | null
+): Component {
+  if (comp.__isBuiltIn) {
+    return comp
+  }
+
+  // 2.x constructor
+  if (isFunction(comp) && comp.cid) {
+    comp = comp.options
+  }
+
+  // 2.x async component
+  if (
+    isFunction(comp) &&
+    checkCompatEnabled(DeprecationTypes.COMPONENT_ASYNC, instance, comp)
+  ) {
+    // since after disabling this, plain functions are still valid usage, do not
+    // use softAssert here.
+    return convertLegacyAsyncComponent(comp)
+  }
+
+  // 2.x functional component
+  if (
+    isObject(comp) &&
+    comp.functional &&
+    softAssertCompatEnabled(
+      DeprecationTypes.COMPONENT_FUNCTIONAL,
+      instance,
+      comp
+    )
+  ) {
+    return convertLegacyFunctionalComponent(comp)
+  }
+
+  return comp
+}
+
+interface LegacyAsyncOptions {
+  component: Promise<Component>
+  loading?: Component
+  error?: Component
+  delay?: number
+  timeout?: number
+}
+
+type LegacyAsyncReturnValue = Promise<Component> | LegacyAsyncOptions
+
+type LegacyAsyncComponent = (
+  resolve?: (res: LegacyAsyncReturnValue) => void,
+  reject?: (reason?: any) => void
+) => LegacyAsyncReturnValue | undefined
+
+const normalizedAsyncComponentMap = new Map<LegacyAsyncComponent, Component>()
+
+function convertLegacyAsyncComponent(comp: LegacyAsyncComponent) {
+  if (normalizedAsyncComponentMap.has(comp)) {
+    return normalizedAsyncComponentMap.get(comp)!
+  }
+
+  // we have to call the function here due to how v2's API won't expose the
+  // options until we call it
+  let resolve: (res: LegacyAsyncReturnValue) => void
+  let reject: (reason?: any) => void
+  const fallbackPromise = new Promise<Component>((r, rj) => {
+    ;(resolve = r), (reject = rj)
+  })
+
+  const res = comp(resolve!, reject!)
+
+  let converted: Component
+  if (isPromise(res)) {
+    converted = defineAsyncComponent(() => res)
+  } else if (isObject(res) && !isVNode(res) && !isArray(res)) {
+    converted = defineAsyncComponent({
+      loader: () => res.component,
+      loadingComponent: res.loading,
+      errorComponent: res.error,
+      delay: res.delay,
+      timeout: res.timeout
+    })
+  } else if (res == null) {
+    converted = defineAsyncComponent(() => fallbackPromise)
+  } else {
+    converted = comp as any // probably a v3 functional comp
+  }
+  normalizedAsyncComponentMap.set(comp, converted)
+  return converted
+}
+
+const normalizedFunctionalComponentMap = new Map<
+  ComponentOptions,
+  FunctionalComponent
+>()
+
+export const legacySlotProxyHandlers: ProxyHandler<InternalSlots> = {
+  get(target, key: string) {
+    const slot = target[key]
+    return slot && slot()
+  }
+}
+
+function convertLegacyFunctionalComponent(comp: ComponentOptions) {
+  if (normalizedFunctionalComponentMap.has(comp)) {
+    return normalizedFunctionalComponentMap.get(comp)!
+  }
+
+  const legacyFn = comp.render as any
+
+  const Func: FunctionalComponent = (props, ctx) => {
+    const instance = getCurrentInstance()!
+
+    const legacyCtx = {
+      props,
+      children: instance.vnode.children || [],
+      data: instance.vnode.props || {},
+      scopedSlots: ctx.slots,
+      parent: instance.parent && instance.parent.proxy,
+      slots() {
+        return new Proxy(ctx.slots, legacySlotProxyHandlers)
+      },
+      get listeners() {
+        return getCompatListeners(instance)
+      },
+      get injections() {
+        if (comp.inject) {
+          const injections = {}
+          resolveInjections(comp.inject, {})
+          return injections
+        }
+        return {}
+      }
+    }
+    return legacyFn(compatH, legacyCtx)
+  }
+  Func.props = comp.props
+  Func.displayName = comp.name
+  // v2 functional components do not inherit attrs
+  Func.inheritAttrs = false
+
+  normalizedFunctionalComponentMap.set(comp, Func)
+  return Func
+}
diff --git a/packages/runtime-core/src/compat/customDirective.ts b/packages/runtime-core/src/compat/customDirective.ts
new file mode 100644
index 00000000..dc854dbc
--- /dev/null
+++ b/packages/runtime-core/src/compat/customDirective.ts
@@ -0,0 +1,60 @@
+import { isArray } from '@vue/shared'
+import { ComponentInternalInstance } from '../component'
+import { ObjectDirective, DirectiveHook } from '../directives'
+import { softAssertCompatEnabled, DeprecationTypes } from './compatConfig'
+
+export interface LegacyDirective {
+  bind?: DirectiveHook
+  inserted?: DirectiveHook
+  update?: DirectiveHook
+  componentUpdated?: DirectiveHook
+  unbind?: DirectiveHook
+}
+
+const legacyDirectiveHookMap: Partial<
+  Record<
+    keyof ObjectDirective,
+    keyof LegacyDirective | (keyof LegacyDirective)[]
+  >
+> = {
+  beforeMount: 'bind',
+  mounted: 'inserted',
+  updated: ['update', 'componentUpdated'],
+  unmounted: 'unbind'
+}
+
+export function mapCompatDirectiveHook(
+  name: keyof ObjectDirective,
+  dir: ObjectDirective & LegacyDirective,
+  instance: ComponentInternalInstance | null
+): DirectiveHook | DirectiveHook[] | undefined {
+  const mappedName = legacyDirectiveHookMap[name]
+  if (mappedName) {
+    if (isArray(mappedName)) {
+      const hook: DirectiveHook[] = []
+      mappedName.forEach(name => {
+        const mappedHook = dir[name]
+        if (mappedHook) {
+          softAssertCompatEnabled(
+            DeprecationTypes.CUSTOM_DIR,
+            instance,
+            mappedName,
+            name
+          )
+          hook.push(mappedHook)
+        }
+      })
+      return hook.length ? hook : undefined
+    } else {
+      if (dir[mappedName]) {
+        softAssertCompatEnabled(
+          DeprecationTypes.CUSTOM_DIR,
+          instance,
+          mappedName,
+          name
+        )
+      }
+      return dir[mappedName]
+    }
+  }
+}
diff --git a/packages/runtime-core/src/compat/data.ts b/packages/runtime-core/src/compat/data.ts
new file mode 100644
index 00000000..121c0f9a
--- /dev/null
+++ b/packages/runtime-core/src/compat/data.ts
@@ -0,0 +1,38 @@
+import { isFunction, isPlainObject } from '@vue/shared'
+import { ComponentInternalInstance } from '../component'
+import { ComponentPublicInstance } from '../componentPublicInstance'
+import { DeprecationTypes, warnDeprecation } from './compatConfig'
+
+export function deepMergeData(
+  to: any,
+  from: any,
+  instance: ComponentInternalInstance
+) {
+  for (const key in from) {
+    const toVal = to[key]
+    const fromVal = from[key]
+    if (key in to && isPlainObject(toVal) && isPlainObject(fromVal)) {
+      __DEV__ &&
+        warnDeprecation(DeprecationTypes.OPTIONS_DATA_MERGE, instance, key)
+      deepMergeData(toVal, fromVal, instance)
+    } else {
+      to[key] = fromVal
+    }
+  }
+}
+
+export function mergeDataOption(to: any, from: any) {
+  if (!from) {
+    return to
+  }
+  if (!to) {
+    return from
+  }
+  return function mergedDataFn(this: ComponentPublicInstance) {
+    return deepMergeData(
+      isFunction(to) ? to.call(this, this) : to,
+      isFunction(from) ? from.call(this, this) : from,
+      this.$
+    )
+  }
+}
diff --git a/packages/runtime-core/src/compat/filter.ts b/packages/runtime-core/src/compat/filter.ts
new file mode 100644
index 00000000..29a5a3fc
--- /dev/null
+++ b/packages/runtime-core/src/compat/filter.ts
@@ -0,0 +1,18 @@
+import { App, AppContext } from '../apiCreateApp'
+import { warn } from '../warning'
+import { assertCompatEnabled, DeprecationTypes } from './compatConfig'
+
+export function installGlobalFilterMethod(app: App, context: AppContext) {
+  context.filters = {}
+  app.filter = (name: string, filter?: Function): any => {
+    assertCompatEnabled(DeprecationTypes.FILTERS, null)
+    if (!filter) {
+      return context.filters![name]
+    }
+    if (__DEV__ && context.filters![name]) {
+      warn(`Filter "${name}" has already been registered.`)
+    }
+    context.filters![name] = filter
+    return app
+  }
+}
diff --git a/packages/runtime-core/src/compat/global.ts b/packages/runtime-core/src/compat/global.ts
new file mode 100644
index 00000000..5dba5f4b
--- /dev/null
+++ b/packages/runtime-core/src/compat/global.ts
@@ -0,0 +1,529 @@
+import {
+  isReactive,
+  reactive,
+  track,
+  TrackOpTypes,
+  trigger,
+  TriggerOpTypes
+} from '@vue/reactivity'
+import {
+  isFunction,
+  extend,
+  NOOP,
+  EMPTY_OBJ,
+  isArray,
+  isObject,
+  isString
+} from '@vue/shared'
+import { warn } from '../warning'
+import { cloneVNode, createVNode } from '../vnode'
+import { RootRenderFunction } from '../renderer'
+import { RootHydrateFunction } from '../hydration'
+import {
+  App,
+  AppConfig,
+  AppContext,
+  CreateAppFunction,
+  Plugin
+} from '../apiCreateApp'
+import {
+  Component,
+  ComponentOptions,
+  createComponentInstance,
+  finishComponentSetup,
+  isRuntimeOnly,
+  setupComponent
+} from '../component'
+import { RenderFunction, mergeOptions } from '../componentOptions'
+import { ComponentPublicInstance } from '../componentPublicInstance'
+import { devtoolsInitApp } from '../devtools'
+import { Directive } from '../directives'
+import { nextTick } from '../scheduler'
+import { version } from '..'
+import { LegacyConfig, legacyOptionMergeStrats } from './globalConfig'
+import { LegacyDirective } from './customDirective'
+import {
+  warnDeprecation,
+  DeprecationTypes,
+  assertCompatEnabled,
+  configureCompat,
+  isCompatEnabled,
+  softAssertCompatEnabled
+} from './compatConfig'
+import { LegacyPublicInstance } from './instance'
+
+/**
+ * @deprecated the default `Vue` export has been removed in Vue 3. The type for
+ * the default export is provided only for migration purposes. Please use
+ * named imports instead - e.g. `import { createApp } from 'vue'`.
+ */
+export type CompatVue = Pick<App, 'version' | 'component' | 'directive'> & {
+  configureCompat: typeof configureCompat
+
+  // no inference here since these types are not meant for actual use - they
+  // are merely here to provide type checks for internal implementation and
+  // information for migration.
+  new (options?: ComponentOptions): LegacyPublicInstance
+
+  version: string
+  config: AppConfig & LegacyConfig
+
+  extend: (options?: ComponentOptions) => CompatVue
+  nextTick: typeof nextTick
+
+  use(plugin: Plugin, ...options: any[]): CompatVue
+  mixin(mixin: ComponentOptions): CompatVue
+
+  component(name: string): Component | undefined
+  component(name: string, component: Component): CompatVue
+  directive(name: string): Directive | undefined
+  directive(name: string, directive: Directive): CompatVue
+
+  compile(template: string): RenderFunction
+
+  /**
+   * @deprecated Vue 3 no longer needs set() for adding new properties.
+   */
+  set(target: any, key: string | number | symbol, value: any): void
+  /**
+   * @deprecated Vue 3 no longer needs delete() for property deletions.
+   */
+  delete(target: any, key: string | number | symbol): void
+  /**
+   * @deprecated use `reactive` instead.
+   */
+  observable: typeof reactive
+  /**
+   * @deprecated filters have been removed from Vue 3.
+   */
+  filter(name: string, arg: any): null
+  /**
+   * @internal
+   */
+  cid: number
+  /**
+   * @internal
+   */
+  options: ComponentOptions
+  /**
+   * @internal
+   */
+  super: CompatVue
+}
+
+export let isCopyingConfig = false
+
+// Legacy global Vue constructor
+export function createCompatVue(
+  createApp: CreateAppFunction<Element>
+): CompatVue {
+  const Vue: CompatVue = function Vue(options: ComponentOptions = {}) {
+    return createCompatApp(options, Vue)
+  } as any
+
+  const singletonApp = createApp({})
+
+  function createCompatApp(options: ComponentOptions = {}, Ctor: any) {
+    assertCompatEnabled(DeprecationTypes.GLOBAL_MOUNT, null)
+
+    const { data } = options
+    if (
+      data &&
+      !isFunction(data) &&
+      softAssertCompatEnabled(DeprecationTypes.OPTIONS_DATA_FN, null)
+    ) {
+      options.data = () => data
+    }
+
+    const app = createApp(options)
+
+    // copy over asset registries and deopt flag
+    ;['mixins', 'components', 'directives', 'deopt'].forEach(key => {
+      // @ts-ignore
+      app._context[key] = singletonApp._context[key]
+    })
+
+    // copy over global config mutations
+    isCopyingConfig = true
+    for (const key in singletonApp.config) {
+      if (key === 'isNativeTag') continue
+      if (
+        isRuntimeOnly() &&
+        (key === 'isCustomElement' || key === 'compilerOptions')
+      ) {
+        continue
+      }
+      const val = singletonApp.config[key as keyof AppConfig]
+      // @ts-ignore
+      app.config[key] = val
+
+      // compat for runtime ignoredElements -> isCustomElement
+      if (
+        key === 'ignoredElements' &&
+        isCompatEnabled(DeprecationTypes.CONFIG_IGNORED_ELEMENTS, null) &&
+        !isRuntimeOnly() &&
+        isArray(val)
+      ) {
+        app.config.compilerOptions.isCustomElement = tag => {
+          return val.some(v => (isString(v) ? v === tag : v.test(tag)))
+        }
+      }
+    }
+    isCopyingConfig = false
+
+    // copy prototype augmentations as config.globalProperties
+    if (isCompatEnabled(DeprecationTypes.GLOBAL_PROTOTYPE, null)) {
+      app.config.globalProperties = Ctor.prototype
+    }
+    let hasPrototypeAugmentations = false
+    for (const key in Ctor.prototype) {
+      if (key !== 'constructor') {
+        hasPrototypeAugmentations = true
+        break
+      }
+    }
+    if (__DEV__ && hasPrototypeAugmentations) {
+      warnDeprecation(DeprecationTypes.GLOBAL_PROTOTYPE, null)
+    }
+
+    const vm = app._createRoot!(options)
+    if (options.el) {
+      return (vm as any).$mount(options.el)
+    } else {
+      return vm
+    }
+  }
+
+  Vue.version = __VERSION__
+  Vue.config = singletonApp.config
+  Vue.nextTick = nextTick
+  Vue.options = { _base: Vue }
+
+  let cid = 1
+  Vue.cid = cid
+
+  const extendCache = new WeakMap()
+
+  function extendCtor(this: any, extendOptions: ComponentOptions = {}) {
+    assertCompatEnabled(DeprecationTypes.GLOBAL_EXTEND, null)
+    if (isFunction(extendOptions)) {
+      extendOptions = extendOptions.options
+    }
+
+    if (extendCache.has(extendOptions)) {
+      return extendCache.get(extendOptions)
+    }
+
+    const Super = this
+    function SubVue(inlineOptions?: ComponentOptions) {
+      if (!inlineOptions) {
+        return createCompatApp(SubVue.options, SubVue)
+      } else {
+        return createCompatApp(
+          mergeOptions(
+            extend({}, SubVue.options),
+            inlineOptions,
+            null,
+            legacyOptionMergeStrats as any
+          ),
+          SubVue
+        )
+      }
+    }
+    SubVue.super = Super
+    SubVue.prototype = Object.create(Vue.prototype)
+    SubVue.prototype.constructor = SubVue
+
+    // clone non-primitive base option values for edge case of mutating
+    // extended options
+    const mergeBase: any = {}
+    for (const key in Super.options) {
+      const superValue = Super.options[key]
+      mergeBase[key] = isArray(superValue)
+        ? superValue.slice()
+        : isObject(superValue)
+          ? extend(Object.create(null), superValue)
+          : superValue
+    }
+
+    SubVue.options = mergeOptions(
+      mergeBase,
+      extendOptions,
+      null,
+      legacyOptionMergeStrats as any
+    )
+
+    SubVue.options._base = SubVue
+    SubVue.extend = extendCtor.bind(SubVue)
+    SubVue.mixin = Super.mixin
+    SubVue.use = Super.use
+    SubVue.cid = ++cid
+
+    extendCache.set(extendOptions, SubVue)
+    return SubVue
+  }
+
+  Vue.extend = extendCtor.bind(Vue) as any
+
+  Vue.set = (target, key, value) => {
+    assertCompatEnabled(DeprecationTypes.GLOBAL_SET, null)
+    target[key] = value
+  }
+
+  Vue.delete = (target, key) => {
+    assertCompatEnabled(DeprecationTypes.GLOBAL_DELETE, null)
+    delete target[key]
+  }
+
+  Vue.observable = (target: any) => {
+    assertCompatEnabled(DeprecationTypes.GLOBAL_OBSERVABLE, null)
+    return reactive(target)
+  }
+
+  Vue.use = (p, ...options) => {
+    if (p && isFunction(p.install)) {
+      p.install(Vue as any, ...options)
+    } else if (isFunction(p)) {
+      p(Vue as any, ...options)
+    }
+    return Vue
+  }
+
+  Vue.mixin = m => {
+    singletonApp.mixin(m)
+    return Vue
+  }
+
+  Vue.component = ((name: string, comp: Component) => {
+    if (comp) {
+      singletonApp.component(name, comp)
+      return Vue
+    } else {
+      return singletonApp.component(name)
+    }
+  }) as any
+
+  Vue.directive = ((name: string, dir: Directive | LegacyDirective) => {
+    if (dir) {
+      singletonApp.directive(name, dir as Directive)
+      return Vue
+    } else {
+      return singletonApp.directive(name)
+    }
+  }) as any
+
+  Vue.filter = ((name: string, filter: any) => {
+    // TODO deprecation warning
+    // TODO compiler warning for filters (maybe behavior compat?)
+  }) as any
+
+  // internal utils - these are technically internal but some plugins use it.
+  const util = {
+    warn: __DEV__ ? warn : NOOP,
+    extend,
+    mergeOptions: (parent: any, child: any, vm?: ComponentPublicInstance) =>
+      mergeOptions(
+        parent,
+        child,
+        vm && vm.$,
+        vm ? undefined : (legacyOptionMergeStrats as any)
+      ),
+    defineReactive
+  }
+  Object.defineProperty(Vue, 'util', {
+    get() {
+      assertCompatEnabled(DeprecationTypes.GLOBAL_PRIVATE_UTIL, null)
+      return util
+    }
+  })
+
+  Vue.configureCompat = configureCompat
+
+  return Vue
+}
+
+export function installCompatMount(
+  app: App,
+  context: AppContext,
+  render: RootRenderFunction,
+  hydrate?: RootHydrateFunction
+) {
+  let isMounted = false
+
+  /**
+   * Vue 2 supports the behavior of creating a component instance but not
+   * mounting it, which is no longer possible in Vue 3 - this internal
+   * function simulates that behavior.
+   */
+  app._createRoot = options => {
+    const component = app._component
+    const vnode = createVNode(component, options.propsData || null)
+    vnode.appContext = context
+
+    const hasNoRender =
+      !isFunction(component) && !component.render && !component.template
+    const emptyRender = () => {}
+
+    // create root instance
+    const instance = createComponentInstance(vnode, null, null)
+    // suppress "missing render fn" warning since it can't be determined
+    // until $mount is called
+    if (hasNoRender) {
+      instance.render = emptyRender
+    }
+    setupComponent(instance)
+    vnode.component = instance
+
+    // $mount & $destroy
+    // these are defined on ctx and picked up by the $mount/$destroy
+    // public property getters on the instance proxy.
+    // Note: the following assumes DOM environment since the compat build
+    // only targets web. It essentially includes logic for app.mount from
+    // both runtime-core AND runtime-dom.
+    instance.ctx._compat_mount = (selectorOrEl?: string | Element) => {
+      if (isMounted) {
+        __DEV__ && warn(`Root instance is already mounted.`)
+        return
+      }
+
+      let container: Element
+      if (typeof selectorOrEl === 'string') {
+        // eslint-disable-next-line
+        const result = document.querySelector(selectorOrEl)
+        if (!result) {
+          __DEV__ &&
+            warn(
+              `Failed to mount root instance: selector "${selectorOrEl}" returned null.`
+            )
+          return
+        }
+        container = result
+      } else {
+        // eslint-disable-next-line
+        container = selectorOrEl || document.createElement('div')
+      }
+
+      const isSVG = container instanceof SVGElement
+
+      // HMR root reload
+      if (__DEV__) {
+        context.reload = () => {
+          const cloned = cloneVNode(vnode)
+          // compat mode will use instance if not reset to null
+          cloned.component = null
+          render(cloned, container, isSVG)
+        }
+      }
+
+      // resolve in-DOM template if component did not provide render
+      // and no setup/mixin render functions are provided (by checking
+      // that the instance is still using the placeholder render fn)
+      if (hasNoRender && instance.render === emptyRender) {
+        // root directives check
+        if (__DEV__) {
+          for (let i = 0; i < container.attributes.length; i++) {
+            const attr = container.attributes[i]
+            if (attr.name !== 'v-cloak' && /^(v-|:|@)/.test(attr.name)) {
+              warnDeprecation(DeprecationTypes.GLOBAL_MOUNT_CONTAINER, null)
+              break
+            }
+          }
+        }
+        instance.render = null
+        ;(component as ComponentOptions).template = container.innerHTML
+        finishComponentSetup(instance, false, true /* skip options */)
+      }
+
+      // clear content before mounting
+      container.innerHTML = ''
+
+      // TODO hydration
+      render(vnode, container, isSVG)
+
+      if (container instanceof Element) {
+        container.removeAttribute('v-cloak')
+        container.setAttribute('data-v-app', '')
+      }
+
+      isMounted = true
+      app._container = container
+      // for devtools and telemetry
+      ;(container as any).__vue_app__ = app
+      if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
+        devtoolsInitApp(app, version)
+      }
+
+      return instance.proxy!
+    }
+
+    instance.ctx._compat_destroy = app.unmount
+
+    return instance.proxy!
+  }
+}
+
+const methodsToPatch = [
+  'push',
+  'pop',
+  'shift',
+  'unshift',
+  'splice',
+  'sort',
+  'reverse'
+]
+
+const patched = new WeakSet<object>()
+
+function defineReactive(obj: any, key: string, val: any) {
+  // it's possible for the orignial object to be mutated after being defined
+  // and expecting reactivity... we are covering it here because this seems to
+  // be a bit more common.
+  if (isObject(val) && !isReactive(val) && !patched.has(val)) {
+    const reactiveVal = reactive(val)
+    if (isArray(val)) {
+      methodsToPatch.forEach(m => {
+        // @ts-ignore
+        val[m] = (...args: any[]) => {
+          // @ts-ignore
+          Array.prototype[m].call(reactiveVal, ...args)
+        }
+      })
+    } else {
+      Object.keys(val).forEach(key => {
+        try {
+          defineReactiveSimple(val, key, val[key])
+        } catch (e) {}
+      })
+    }
+  }
+
+  const i = obj.$
+  if (i && obj === i.proxy) {
+    // Vue instance, add it to data
+    if (i.data === EMPTY_OBJ) {
+      i.data = reactive({})
+    }
+    i.data[key] = val
+    i.accessCache = Object.create(null)
+  } else if (isReactive(obj)) {
+    obj[key] = val
+  } else {
+    defineReactiveSimple(obj, key, val)
+  }
+}
+
+function defineReactiveSimple(obj: any, key: string, val: any) {
+  val = isObject(val) ? reactive(val) : val
+  Object.defineProperty(obj, key, {
+    enumerable: true,
+    configurable: true,
+    get() {
+      track(obj, TrackOpTypes.GET, key)
+      return val
+    },
+    set(newVal) {
+      val = isObject(newVal) ? reactive(newVal) : newVal
+      trigger(obj, TriggerOpTypes.SET, key, newVal)
+    }
+  })
+}
diff --git a/packages/runtime-core/src/compat/globalConfig.ts b/packages/runtime-core/src/compat/globalConfig.ts
new file mode 100644
index 00000000..cd5b4194
--- /dev/null
+++ b/packages/runtime-core/src/compat/globalConfig.ts
@@ -0,0 +1,105 @@
+import { extend, isArray } from '@vue/shared'
+import { AppConfig } from '../apiCreateApp'
+import { mergeDataOption } from './data'
+import { DeprecationTypes, warnDeprecation } from './compatConfig'
+import { isCopyingConfig } from './global'
+
+// legacy config warnings
+export type LegacyConfig = {
+  /**
+   * @deprecated `config.silent` option has been removed
+   */
+  silent?: boolean
+  /**
+   * @deprecated use __VUE_PROD_DEVTOOLS__ compile-time feature flag instead
+   * https://github.com/vuejs/vue-next/tree/master/packages/vue#bundler-build-feature-flags
+   */
+  devtools?: boolean
+  /**
+   * @deprecated use `config.isCustomElement` instead
+   * https://v3.vuejs.org/guide/migration/global-api.html#config-ignoredelements-is-now-config-iscustomelement
+   */
+  ignoredElements?: (string | RegExp)[]
+  /**
+   * @deprecated
+   * https://v3.vuejs.org/guide/migration/keycode-modifiers.html
+   */
+  keyCodes?: Record<string, number | number[]>
+  /**
+   * @deprecated
+   * https://v3.vuejs.org/guide/migration/global-api.html#config-productiontip-removed
+   */
+  productionTip?: boolean
+}
+
+// dev only
+export function installLegacyConfigProperties(config: AppConfig) {
+  const legacyConfigOptions: Record<string, DeprecationTypes> = {
+    silent: DeprecationTypes.CONFIG_SILENT,
+    devtools: DeprecationTypes.CONFIG_DEVTOOLS,
+    ignoredElements: DeprecationTypes.CONFIG_IGNORED_ELEMENTS,
+    keyCodes: DeprecationTypes.CONFIG_KEY_CODES,
+    productionTip: DeprecationTypes.CONFIG_PRODUCTION_TIP
+  }
+
+  Object.keys(legacyConfigOptions).forEach(key => {
+    let val = (config as any)[key]
+    Object.defineProperty(config, key, {
+      enumerable: true,
+      get() {
+        return val
+      },
+      set(newVal) {
+        if (!isCopyingConfig) {
+          warnDeprecation(legacyConfigOptions[key], null)
+        }
+        val = newVal
+      }
+    })
+  })
+
+  // Internal merge strats which are no longer needed in v3, but we need to
+  // expose them because some v2 plugins will reuse these internal strats to
+  // merge their custom options.
+  extend(config.optionMergeStrategies, legacyOptionMergeStrats)
+}
+
+export const legacyOptionMergeStrats = {
+  data: mergeDataOption,
+  beforeCreate: mergeHook,
+  created: mergeHook,
+  beforeMount: mergeHook,
+  mounted: mergeHook,
+  beforeUpdate: mergeHook,
+  updated: mergeHook,
+  beforeDestroy: mergeHook,
+  destroyed: mergeHook,
+  activated: mergeHook,
+  deactivated: mergeHook,
+  errorCaptured: mergeHook,
+  serverPrefetch: mergeHook,
+  // assets
+  components: mergeObjectOptions,
+  directives: mergeObjectOptions,
+  filters: mergeObjectOptions,
+  // objects
+  props: mergeObjectOptions,
+  methods: mergeObjectOptions,
+  inject: mergeObjectOptions,
+  computed: mergeObjectOptions,
+  // watch has special merge behavior in v2, but isn't actually needed in v3.
+  // since we are only exposing these for compat and nobody should be relying
+  // on the watch-specific behavior, just expose the object merge strat.
+  watch: mergeObjectOptions
+}
+
+function mergeHook(
+  to: Function[] | Function | undefined,
+  from: Function | Function[]
+) {
+  return Array.from(new Set([...(isArray(to) ? to : to ? [to] : []), from]))
+}
+
+function mergeObjectOptions(to: Object | undefined, from: Object | undefined) {
+  return to ? extend(extend(Object.create(null), to), from) : from
+}
diff --git a/packages/runtime-core/src/compat/instance.ts b/packages/runtime-core/src/compat/instance.ts
new file mode 100644
index 00000000..10c59ceb
--- /dev/null
+++ b/packages/runtime-core/src/compat/instance.ts
@@ -0,0 +1,154 @@
+import {
+  extend,
+  looseEqual,
+  looseIndexOf,
+  NOOP,
+  toDisplayString,
+  toNumber
+} from '@vue/shared'
+import {
+  ComponentPublicInstance,
+  PublicPropertiesMap
+} from '../componentPublicInstance'
+import { getCompatChildren } from './instanceChildren'
+import {
+  DeprecationTypes,
+  assertCompatEnabled,
+  isCompatEnabled
+} from './compatConfig'
+import { off, on, once } from './instanceEventEmitter'
+import { getCompatListeners } from './instanceListeners'
+import { shallowReadonly } from '@vue/reactivity'
+import { legacySlotProxyHandlers } from './component'
+import { compatH } from './renderFn'
+import { createCommentVNode, createTextVNode } from '../vnode'
+import { renderList } from '../helpers/renderList'
+import {
+  legacyBindDynamicKeys,
+  legacyBindObjectListeners,
+  legacyBindObjectProps,
+  legacyCheckKeyCodes,
+  legacyMarkOnce,
+  legacyPrependModifier,
+  legacyRenderSlot,
+  legacyRenderStatic,
+  legacyresolveScopedSlots
+} from './renderHelpers'
+import { resolveFilter } from '../helpers/resolveAssets'
+import { resolveMergedOptions } from '../componentOptions'
+import { Slots } from '../componentSlots'
+
+export type LegacyPublicInstance = ComponentPublicInstance &
+  LegacyPublicProperties
+
+export interface LegacyPublicProperties {
+  $set(target: object, key: string, value: any): void
+  $delete(target: object, key: string): void
+  $mount(el?: string | Element): this
+  $destroy(): void
+  $scopedSlots: Slots
+  $on(event: string | string[], fn: Function): this
+  $once(event: string, fn: Function): this
+  $off(event?: string, fn?: Function): this
+  $children: LegacyPublicProperties[]
+  $listeners: Record<string, Function | Function[]>
+}
+
+export function installCompatInstanceProperties(map: PublicPropertiesMap) {
+  const set = (target: any, key: any, val: any) => {
+    target[key] = val
+  }
+
+  const del = (target: any, key: any) => {
+    delete target[key]
+  }
+
+  extend(map, {
+    $set: i => {
+      assertCompatEnabled(DeprecationTypes.INSTANCE_SET, i)
+      return set
+    },
+
+    $delete: i => {
+      assertCompatEnabled(DeprecationTypes.INSTANCE_DELETE, i)
+      return del
+    },
+
+    $mount: i => {
+      assertCompatEnabled(
+        DeprecationTypes.GLOBAL_MOUNT,
+        null /* this warning is global */
+      )
+      // root mount override from ./global.ts in installCompatMount
+      return i.ctx._compat_mount || NOOP
+    },
+
+    $destroy: i => {
+      assertCompatEnabled(DeprecationTypes.INSTANCE_DESTROY, i)
+      // root destroy override from ./global.ts in installCompatMount
+      return i.ctx._compat_destroy || NOOP
+    },
+
+    // overrides existing accessor
+    $slots: i => {
+      if (
+        isCompatEnabled(DeprecationTypes.RENDER_FUNCTION, i) &&
+        i.render &&
+        i.render._compatWrapped
+      ) {
+        return new Proxy(i.slots, legacySlotProxyHandlers)
+      }
+      return __DEV__ ? shallowReadonly(i.slots) : i.slots
+    },
+
+    $scopedSlots: i => {
+      assertCompatEnabled(DeprecationTypes.INSTANCE_SCOPED_SLOTS, i)
+      return __DEV__ ? shallowReadonly(i.slots) : i.slots
+    },
+
+    $on: i => on.bind(null, i),
+    $once: i => once.bind(null, i),
+    $off: i => off.bind(null, i),
+
+    $children: getCompatChildren,
+    $listeners: getCompatListeners
+  } as PublicPropertiesMap)
+
+  if (isCompatEnabled(DeprecationTypes.PRIVATE_APIS, null)) {
+    extend(map, {
+      $vnode: i => i.vnode,
+
+      // inject addtional properties into $options for compat
+      $options: i => {
+        let res = resolveMergedOptions(i)
+        if (res === i.type) res = i.type.__merged = extend({}, res)
+        res.parent = i.proxy!.$parent
+        res.propsData = i.vnode.props
+        return res
+      },
+
+      // v2 render helpers
+      $createElement: () => compatH,
+      _self: i => i.proxy,
+      _uid: i => i.uid,
+      _c: () => compatH,
+      _o: () => legacyMarkOnce,
+      _n: () => toNumber,
+      _s: () => toDisplayString,
+      _l: () => renderList,
+      _t: i => legacyRenderSlot.bind(null, i),
+      _q: () => looseEqual,
+      _i: () => looseIndexOf,
+      _m: i => legacyRenderStatic.bind(null, i),
+      _f: () => resolveFilter,
+      _k: i => legacyCheckKeyCodes.bind(null, i),
+      _b: () => legacyBindObjectProps,
+      _v: () => createTextVNode,
+      _e: () => createCommentVNode,
+      _u: () => legacyresolveScopedSlots,
+      _g: () => legacyBindObjectListeners,
+      _d: () => legacyBindDynamicKeys,
+      _p: () => legacyPrependModifier
+    } as PublicPropertiesMap)
+  }
+}
diff --git a/packages/runtime-core/src/compat/instanceChildren.ts b/packages/runtime-core/src/compat/instanceChildren.ts
new file mode 100644
index 00000000..349b5a3f
--- /dev/null
+++ b/packages/runtime-core/src/compat/instanceChildren.ts
@@ -0,0 +1,28 @@
+import { ShapeFlags } from '@vue/shared/src'
+import { ComponentInternalInstance } from '../component'
+import { ComponentPublicInstance } from '../componentPublicInstance'
+import { VNode } from '../vnode'
+import { assertCompatEnabled, DeprecationTypes } from './compatConfig'
+
+export function getCompatChildren(
+  instance: ComponentInternalInstance
+): ComponentPublicInstance[] {
+  assertCompatEnabled(DeprecationTypes.INSTANCE_CHILDREN, instance)
+  const root = instance.subTree
+  const children: ComponentPublicInstance[] = []
+  if (root) {
+    walk(root, children)
+  }
+  return children
+}
+
+function walk(vnode: VNode, children: ComponentPublicInstance[]) {
+  if (vnode.component) {
+    children.push(vnode.component.proxy!)
+  } else if (vnode.shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
+    const vnodes = vnode.children as VNode[]
+    for (let i = 0; i < vnodes.length; i++) {
+      walk(vnodes[i], children)
+    }
+  }
+}
diff --git a/packages/runtime-core/src/compat/instanceEventEmitter.ts b/packages/runtime-core/src/compat/instanceEventEmitter.ts
new file mode 100644
index 00000000..64237e91
--- /dev/null
+++ b/packages/runtime-core/src/compat/instanceEventEmitter.ts
@@ -0,0 +1,108 @@
+import { isArray } from '@vue/shared'
+import { ComponentInternalInstance } from '../component'
+import { callWithAsyncErrorHandling, ErrorCodes } from '../errorHandling'
+import { assertCompatEnabled, DeprecationTypes } from './compatConfig'
+
+interface EventRegistry {
+  [event: string]: Function[] | undefined
+}
+
+const eventRegistryMap = /*#__PURE__*/ new WeakMap<
+  ComponentInternalInstance,
+  EventRegistry
+>()
+
+export function getRegistry(
+  instance: ComponentInternalInstance
+): EventRegistry {
+  let events = eventRegistryMap.get(instance)
+  if (!events) {
+    eventRegistryMap.set(instance, (events = Object.create(null)))
+  }
+  return events!
+}
+
+export function on(
+  instance: ComponentInternalInstance,
+  event: string | string[],
+  fn: Function
+) {
+  if (isArray(event)) {
+    event.forEach(e => on(instance, e, fn))
+  } else {
+    if (event.startsWith('hook:')) {
+      assertCompatEnabled(
+        DeprecationTypes.INSTANCE_EVENT_HOOKS,
+        instance,
+        event
+      )
+    } else {
+      assertCompatEnabled(DeprecationTypes.INSTANCE_EVENT_EMITTER, instance)
+    }
+    const events = getRegistry(instance)
+    ;(events[event] || (events[event] = [])).push(fn)
+  }
+  return instance.proxy
+}
+
+export function once(
+  instance: ComponentInternalInstance,
+  event: string,
+  fn: Function
+) {
+  const wrapped = (...args: any[]) => {
+    off(instance, event, wrapped)
+    fn.call(instance.proxy, ...args)
+  }
+  wrapped.fn = fn
+  on(instance, event, wrapped)
+  return instance.proxy
+}
+
+export function off(
+  instance: ComponentInternalInstance,
+  event?: string,
+  fn?: Function
+) {
+  assertCompatEnabled(DeprecationTypes.INSTANCE_EVENT_EMITTER, instance)
+  const vm = instance.proxy
+  // all
+  if (!arguments.length) {
+    eventRegistryMap.set(instance, Object.create(null))
+    return vm
+  }
+  // array of events
+  if (isArray(event)) {
+    event.forEach(e => off(instance, e, fn))
+    return vm
+  }
+  // specific event
+  const events = getRegistry(instance)
+  const cbs = events[event!]
+  if (!cbs) {
+    return vm
+  }
+  if (!fn) {
+    events[event!] = undefined
+    return vm
+  }
+  events[event!] = cbs.filter(cb => !(cb === fn || (cb as any).fn === fn))
+  return vm
+}
+
+export function emit(
+  instance: ComponentInternalInstance,
+  event: string,
+  ...args: any[]
+) {
+  const cbs = getRegistry(instance)[event]
+  if (cbs) {
+    callWithAsyncErrorHandling(
+      cbs,
+      instance,
+      ErrorCodes.COMPONENT_EVENT_HANDLER,
+      args
+    )
+  }
+  return instance.proxy
+}
diff --git a/packages/runtime-core/src/compat/instanceListeners.ts b/packages/runtime-core/src/compat/instanceListeners.ts
new file mode 100644
index 00000000..973d440b
--- /dev/null
+++ b/packages/runtime-core/src/compat/instanceListeners.ts
@@ -0,0 +1,19 @@
+import { isOn } from '@vue/shared'
+import { ComponentInternalInstance } from '../component'
+import { assertCompatEnabled, DeprecationTypes } from './compatConfig'
+
+export function getCompatListeners(instance: ComponentInternalInstance) {
+  assertCompatEnabled(DeprecationTypes.INSTANCE_LISTENERS, instance)
+
+  const listeners: Record<string, Function | Function[]> = {}
+  const rawProps = instance.vnode.props
+  if (!rawProps) {
+    return listeners
+  }
+  for (const key in rawProps) {
+    if (isOn(key)) {
+      listeners[key[2].toLowerCase() + key.slice(3)] = rawProps[key]
+    }
+  }
+  return listeners
+}
diff --git a/packages/runtime-core/src/compat/props.ts b/packages/runtime-core/src/compat/props.ts
new file mode 100644
index 00000000..e2fccb66
--- /dev/null
+++ b/packages/runtime-core/src/compat/props.ts
@@ -0,0 +1,40 @@
+import { isArray } from '@vue/shared'
+import { inject } from '../apiInject'
+import { ComponentInternalInstance, Data } from '../component'
+import { ComponentOptions, resolveMergedOptions } from '../componentOptions'
+import { DeprecationTypes, warnDeprecation } from './compatConfig'
+
+export function createPropsDefaultThis(
+  instance: ComponentInternalInstance,
+  rawProps: Data,
+  propKey: string
+) {
+  return new Proxy(
+    {},
+    {
+      get(_, key: string) {
+        __DEV__ &&
+          warnDeprecation(DeprecationTypes.PROPS_DEFAULT_THIS, null, propKey)
+        // $options
+        if (key === '$options') {
+          return resolveMergedOptions(instance)
+        }
+        // props
+        if (key in rawProps) {
+          return rawProps[key]
+        }
+        // injections
+        const injections = (instance.type as ComponentOptions).inject
+        if (injections) {
+          if (isArray(injections)) {
+            if (injections.includes(key)) {
+              return inject(key)
+            }
+          } else if (key in injections) {
+            return inject(key)
+          }
+        }
+      }
+    }
+  )
+}
diff --git a/packages/runtime-core/src/compat/ref.ts b/packages/runtime-core/src/compat/ref.ts
new file mode 100644
index 00000000..7b827121
--- /dev/null
+++ b/packages/runtime-core/src/compat/ref.ts
@@ -0,0 +1,45 @@
+import { isArray, remove } from '@vue/shared'
+import { ComponentInternalInstance, Data } from '../component'
+import { VNode } from '../vnode'
+import { DeprecationTypes, warnDeprecation } from './compatConfig'
+
+export function convertLegacyRefInFor(vnode: VNode) {
+  // refInFor
+  if (vnode.props && vnode.props.refInFor) {
+    delete vnode.props.refInFor
+    if (vnode.ref) {
+      if (isArray(vnode.ref)) {
+        vnode.ref.forEach(r => (r.f = true))
+      } else {
+        vnode.ref.f = true
+      }
+    }
+  }
+}
+
+export function registerLegacyRef(
+  refs: Data,
+  key: string,
+  value: any,
+  owner: ComponentInternalInstance,
+  isInFor: boolean | undefined,
+  isUnmount: boolean
+) {
+  const existing = refs[key]
+  if (isUnmount) {
+    if (isArray(existing)) {
+      remove(existing, value)
+    } else {
+      refs[key] = null
+    }
+  } else if (isInFor) {
+    __DEV__ && warnDeprecation(DeprecationTypes.V_FOR_REF, owner)
+    if (!isArray(existing)) {
+      refs[key] = [value]
+    } else if (!existing.includes(value)) {
+      existing.push(value)
+    }
+  } else {
+    refs[key] = value
+  }
+}
diff --git a/packages/runtime-core/src/compat/renderFn.ts b/packages/runtime-core/src/compat/renderFn.ts
new file mode 100644
index 00000000..c4dbcf29
--- /dev/null
+++ b/packages/runtime-core/src/compat/renderFn.ts
@@ -0,0 +1,344 @@
+import {
+  extend,
+  hyphenate,
+  isArray,
+  isObject,
+  isString,
+  makeMap,
+  normalizeClass,
+  normalizeStyle,
+  ShapeFlags,
+  toHandlerKey
+} from '@vue/shared'
+import {
+  Component,
+  ComponentInternalInstance,
+  ComponentOptions,
+  Data,
+  InternalRenderFunction
+} from '../component'
+import { currentRenderingInstance } from '../componentRenderContext'
+import { DirectiveArguments, withDirectives } from '../directives'
+import {
+  resolveDirective,
+  resolveDynamicComponent
+} from '../helpers/resolveAssets'
+import {
+  Comment,
+  createVNode,
+  isVNode,
+  normalizeChildren,
+  VNode,
+  VNodeArrayChildren,
+  VNodeProps
+} from '../vnode'
+import {
+  checkCompatEnabled,
+  DeprecationTypes,
+  isCompatEnabled
+} from './compatConfig'
+import { compatModelEventPrefix } from './vModel'
+
+const v3CompiledRenderFnRE = /^(?:function \w+)?\(_ctx, _cache/
+
+export function convertLegacyRenderFn(instance: ComponentInternalInstance) {
+  const Component = instance.type as ComponentOptions
+  const render = Component.render as InternalRenderFunction | undefined
+
+  // v3 runtime compiled, or already checked / wrapped
+  if (!render || render._rc || render._compatChecked || render._compatWrapped) {
+    return
+  }
+
+  if (v3CompiledRenderFnRE.test(render.toString())) {
+    // v3 pre-compiled function
+    render._compatChecked = true
+    return
+  }
+
+  // v2 render function, try to provide compat
+  if (checkCompatEnabled(DeprecationTypes.RENDER_FUNCTION, instance)) {
+    const wrapped = (Component.render = function compatRender() {
+      // @ts-ignore
+      return render.call(this, compatH)
+    })
+    // @ts-ignore
+    wrapped._compatWrapped = true
+  }
+}
+
+interface LegacyVNodeProps {
+  key?: string | number
+  ref?: string
+  refInFor?: boolean
+
+  staticClass?: string
+  class?: unknown
+  staticStyle?: Record<string, unknown>
+  style?: Record<string, unknown>
+  attrs?: Record<string, unknown>
+  domProps?: Record<string, unknown>
+  on?: Record<string, Function | Function[]>
+  nativeOn?: Record<string, Function | Function[]>
+  directives?: LegacyVNodeDirective[]
+
+  // component only
+  props?: Record<string, unknown>
+  slot?: string
+  scopedSlots?: Record<string, Function>
+  model?: {
+    value: any
+    callback: (v: any) => void
+    expression: string
+  }
+}
+
+interface LegacyVNodeDirective {
+  name: string
+  value: unknown
+  arg?: string
+  modifiers?: Record<string, boolean>
+}
+
+type LegacyVNodeChildren =
+  | string
+  | number
+  | boolean
+  | VNode
+  | VNodeArrayChildren
+
+export function compatH(
+  type: string | Component,
+  children?: LegacyVNodeChildren
+): VNode
+export function compatH(
+  type: string | Component,
+  props?: LegacyVNodeProps,
+  children?: LegacyVNodeChildren
+): VNode
+
+export function compatH(
+  type: any,
+  propsOrChildren?: any,
+  children?: any
+): VNode {
+  if (!type) {
+    type = Comment
+  }
+
+  // to support v2 string component name look!up
+  if (typeof type === 'string') {
+    const t = hyphenate(type)
+    if (t === 'transition' || t === 'transition-group' || t === 'keep-alive') {
+      // since transition and transition-group are runtime-dom-specific,
+      // we cannot import them directly here. Instead they are registered using
+      // special keys in @vue/compat entry.
+      type = `__compat__${t}`
+    }
+    type = resolveDynamicComponent(type)
+  }
+
+  const l = arguments.length
+  const is2ndArgArrayChildren = isArray(propsOrChildren)
+  if (l === 2 || is2ndArgArrayChildren) {
+    if (isObject(propsOrChildren) && !is2ndArgArrayChildren) {
+      // single vnode without props
+      if (isVNode(propsOrChildren)) {
+        return convertLegacySlots(createVNode(type, null, [propsOrChildren]))
+      }
+      // props without children
+      return convertLegacySlots(
+        convertLegacyDirectives(
+          createVNode(type, convertLegacyProps(propsOrChildren, type)),
+          propsOrChildren
+        )
+      )
+    } else {
+      // omit props
+      return convertLegacySlots(createVNode(type, null, propsOrChildren))
+    }
+  } else {
+    if (isVNode(children)) {
+      children = [children]
+    }
+    return convertLegacySlots(
+      convertLegacyDirectives(
+        createVNode(type, convertLegacyProps(propsOrChildren, type), children),
+        propsOrChildren
+      )
+    )
+  }
+}
+
+const skipLegacyRootLevelProps = /*#__PURE__*/ makeMap(
+  'staticStyle,staticClass,directives,model,hook'
+)
+
+function convertLegacyProps(
+  legacyProps: LegacyVNodeProps | undefined,
+  type: any
+): Data & VNodeProps | null {
+  if (!legacyProps) {
+    return null
+  }
+
+  const converted: Data & VNodeProps = {}
+
+  for (const key in legacyProps) {
+    if (key === 'attrs' || key === 'domProps' || key === 'props') {
+      extend(converted, legacyProps[key])
+    } else if (key === 'on' || key === 'nativeOn') {
+      const listeners = legacyProps[key]
+      for (const event in listeners) {
+        const handlerKey = convertLegacyEventKey(event)
+        const existing = converted[handlerKey]
+        const incoming = listeners[event]
+        if (existing !== incoming) {
+          if (existing) {
+            // for the rare case where the same handler is attached
+            // twice with/without .native modifier...
+            if (key === 'nativeOn' && String(existing) === String(incoming)) {
+              continue
+            }
+            converted[handlerKey] = [].concat(existing as any, incoming as any)
+          } else {
+            converted[handlerKey] = incoming
+          }
+        }
+      }
+    } else if (!skipLegacyRootLevelProps(key)) {
+      converted[key] = legacyProps[key as keyof LegacyVNodeProps]
+    }
+  }
+
+  if (legacyProps.staticClass) {
+    converted.class = normalizeClass([legacyProps.staticClass, converted.class])
+  }
+  if (legacyProps.staticStyle) {
+    converted.style = normalizeStyle([legacyProps.staticStyle, converted.style])
+  }
+
+  if (legacyProps.model && isObject(type)) {
+    // v2 compiled component v-model
+    const { prop = 'value', event = 'input' } = (type as any).model || {}
+    converted[prop] = legacyProps.model.value
+    converted[compatModelEventPrefix + event] = legacyProps.model.callback
+  }
+
+  return converted
+}
+
+function convertLegacyEventKey(event: string): string {
+  // normalize v2 event prefixes
+  if (event[0] === '&') {
+    event = event.slice(1) + 'Passive'
+  }
+  if (event[0] === '~') {
+    event = event.slice(1) + 'Once'
+  }
+  if (event[0] === '!') {
+    event = event.slice(1) + 'Capture'
+  }
+  return toHandlerKey(event)
+}
+
+function convertLegacyDirectives(
+  vnode: VNode,
+  props?: LegacyVNodeProps
+): VNode {
+  if (props && props.directives) {
+    return withDirectives(
+      vnode,
+      props.directives.map(({ name, value, arg, modifiers }) => {
+        return [
+          resolveDirective(name)!,
+          value,
+          arg,
+          modifiers
+        ] as DirectiveArguments[number]
+      })
+    )
+  }
+  return vnode
+}
+
+function convertLegacySlots(vnode: VNode): VNode {
+  const { props, children } = vnode
+
+  let slots: Record<string, any> | undefined
+
+  if (vnode.shapeFlag & ShapeFlags.COMPONENT && isArray(children)) {
+    slots = {}
+    // check "slot" property on vnodes and turn them into v3 function slots
+    for (let i = 0; i < children.length; i++) {
+      const child = children[i]
+      const slotName =
+        (isVNode(child) && child.props && child.props.slot) || 'default'
+      const slot = slots[slotName] || (slots[slotName] = [] as any[])
+      if (isVNode(child) && child.type === 'template') {
+        slot.push(child.children)
+      } else {
+        slot.push(child)
+      }
+    }
+    if (slots) {
+      for (const key in slots) {
+        const slotChildren = slots[key]
+        slots[key] = () => slotChildren
+      }
+    }
+  }
+
+  const scopedSlots = props && props.scopedSlots
+  if (scopedSlots) {
+    delete props!.scopedSlots
+    if (slots) {
+      extend(slots, scopedSlots)
+    } else {
+      slots = scopedSlots
+    }
+  }
+
+  if (slots) {
+    normalizeChildren(vnode, slots)
+  }
+
+  return vnode
+}
+
+export function defineLegacyVNodeProperties(vnode: VNode) {
+  if (
+    isCompatEnabled(
+      DeprecationTypes.RENDER_FUNCTION,
+      currentRenderingInstance
+    ) &&
+    isCompatEnabled(DeprecationTypes.PRIVATE_APIS, currentRenderingInstance)
+  ) {
+    const context = currentRenderingInstance
+    const getInstance = () => vnode.component && vnode.component.proxy
+    let componentOptions: any
+    Object.defineProperties(vnode, {
+      tag: { get: () => vnode.type },
+      data: { get: () => vnode.props, set: p => (vnode.props = p) },
+      elm: { get: () => vnode.el },
+      componentInstance: { get: getInstance },
+      child: { get: getInstance },
+      text: { get: () => (isString(vnode.children) ? vnode.children : null) },
+      context: { get: () => context && context.proxy },
+      componentOptions: {
+        get: () => {
+          if (vnode.shapeFlag & ShapeFlags.STATEFUL_COMPONENT) {
+            if (componentOptions) {
+              return componentOptions
+            }
+            return (componentOptions = {
+              Ctor: vnode.type,
+              propsData: vnode.props,
+              children: vnode.children
+            })
+          }
+        }
+      }
+    })
+  }
+}
diff --git a/packages/runtime-core/src/compat/renderHelpers.ts b/packages/runtime-core/src/compat/renderHelpers.ts
new file mode 100644
index 00000000..2eaaa459
--- /dev/null
+++ b/packages/runtime-core/src/compat/renderHelpers.ts
@@ -0,0 +1,182 @@
+import {
+  camelize,
+  extend,
+  hyphenate,
+  isArray,
+  isObject,
+  isReservedProp,
+  normalizeClass
+} from '@vue/shared'
+import { ComponentInternalInstance } from '../component'
+import { Slot } from '../componentSlots'
+import { createSlots } from '../helpers/createSlots'
+import { renderSlot } from '../helpers/renderSlot'
+import { toHandlers } from '../helpers/toHandlers'
+import { mergeProps, VNode } from '../vnode'
+
+function toObject(arr: Array<any>): Object {
+  const res = {}
+  for (let i = 0; i < arr.length; i++) {
+    if (arr[i]) {
+      extend(res, arr[i])
+    }
+  }
+  return res
+}
+
+export function legacyBindObjectProps(
+  data: any,
+  _tag: string,
+  value: any,
+  _asProp: boolean,
+  isSync?: boolean
+) {
+  if (value && isObject(value)) {
+    if (isArray(value)) {
+      value = toObject(value)
+    }
+    for (const key in value) {
+      if (isReservedProp(key)) {
+        data[key] = value[key]
+      } else if (key === 'class') {
+        data.class = normalizeClass([data.class, value.class])
+      } else if (key === 'style') {
+        data.style = normalizeClass([data.style, value.style])
+      } else {
+        const attrs = data.attrs || (data.attrs = {})
+        const camelizedKey = camelize(key)
+        const hyphenatedKey = hyphenate(key)
+        if (!(camelizedKey in attrs) && !(hyphenatedKey in attrs)) {
+          attrs[key] = value[key]
+
+          if (isSync) {
+            const on = data.on || (data.on = {})
+            on[`update:${key}`] = function($event: any) {
+              value[key] = $event
+            }
+          }
+        }
+      }
+    }
+  }
+  return data
+}
+
+export function legacyBindObjectListeners(props: any, listeners: any) {
+  return mergeProps(props, toHandlers(listeners))
+}
+
+export function legacyRenderSlot(
+  instance: ComponentInternalInstance,
+  name: string,
+  fallback?: VNode[],
+  props?: any,
+  bindObject?: any
+) {
+  if (bindObject) {
+    props = mergeProps(props, bindObject)
+  }
+  return renderSlot(instance.slots, name, props, fallback && (() => fallback))
+}
+
+type LegacyScopedSlotsData = Array<
+  | {
+      key: string
+      fn: Function
+    }
+  | LegacyScopedSlotsData
+>
+
+export function legacyresolveScopedSlots(
+  fns: LegacyScopedSlotsData,
+  raw?: Record<string, Slot>,
+  // the following are added in 2.6
+  hasDynamicKeys?: boolean
+) {
+  // v2 default slot doesn't have name
+  return createSlots(
+    raw || ({ $stable: !hasDynamicKeys } as any),
+    mapKeyToName(fns)
+  )
+}
+
+function mapKeyToName(slots: LegacyScopedSlotsData) {
+  for (let i = 0; i < slots.length; i++) {
+    const fn = slots[i]
+    if (fn) {
+      if (isArray(fn)) {
+        mapKeyToName(fn)
+      } else {
+        ;(fn as any).name = fn.key || 'default'
+      }
+    }
+  }
+  return slots as any
+}
+
+const staticCacheMap = /*#__PURE__*/ new WeakMap<
+  ComponentInternalInstance,
+  any[]
+>()
+
+export function legacyRenderStatic(
+  instance: ComponentInternalInstance,
+  index: number
+) {
+  let cache = staticCacheMap.get(instance)
+  if (!cache) {
+    staticCacheMap.set(instance, (cache = []))
+  }
+  if (cache[index]) {
+    return cache[index]
+  }
+  const fn = (instance.type as any).staticRenderFns[index]
+  const ctx = instance.proxy
+  return (cache[index] = fn.call(ctx, null, ctx))
+}
+
+export function legacyCheckKeyCodes(
+  instance: ComponentInternalInstance,
+  eventKeyCode: number,
+  key: string,
+  builtInKeyCode?: number | number[],
+  eventKeyName?: string,
+  builtInKeyName?: string | string[]
+) {
+  const config = instance.appContext.config as any
+  const configKeyCodes = config.keyCodes || {}
+  const mappedKeyCode = configKeyCodes[key] || builtInKeyCode
+  if (builtInKeyName && eventKeyName && !configKeyCodes[key]) {
+    return isKeyNotMatch(builtInKeyName, eventKeyName)
+  } else if (mappedKeyCode) {
+    return isKeyNotMatch(mappedKeyCode, eventKeyCode)
+  } else if (eventKeyName) {
+    return hyphenate(eventKeyName) !== key
+  }
+}
+
+function isKeyNotMatch<T>(expect: T | T[], actual: T): boolean {
+  if (isArray(expect)) {
+    return expect.indexOf(actual) === -1
+  } else {
+    return expect !== actual
+  }
+}
+
+export function legacyMarkOnce(tree: VNode) {
+  return tree
+}
+
+export function legacyBindDynamicKeys(props: any, values: any[]) {
+  for (let i = 0; i < values.length; i += 2) {
+    const key = values[i]
+    if (typeof key === 'string' && key) {
+      props[values[i]] = values[i + 1]
+    }
+  }
+  return props
+}
+
+export function legacyPrependModifier(value: any, symbol: string) {
+  return typeof value === 'string' ? symbol + value : value
+}
diff --git a/packages/runtime-core/src/compat/vModel.ts b/packages/runtime-core/src/compat/vModel.ts
new file mode 100644
index 00000000..03ae787a
--- /dev/null
+++ b/packages/runtime-core/src/compat/vModel.ts
@@ -0,0 +1,71 @@
+import { ShapeFlags } from '@vue/shared'
+import { ComponentInternalInstance, ComponentOptions } from '../component'
+import { callWithErrorHandling, ErrorCodes } from '../errorHandling'
+import { VNode } from '../vnode'
+import { popWarningContext, pushWarningContext } from '../warning'
+import {
+  DeprecationTypes,
+  warnDeprecation,
+  isCompatEnabled
+} from './compatConfig'
+
+export const compatModelEventPrefix = `onModelCompat:`
+
+const warnedTypes = new WeakSet()
+
+export function convertLegacyVModelProps(vnode: VNode) {
+  const { type, shapeFlag, props, dynamicProps } = vnode
+  if (shapeFlag & ShapeFlags.COMPONENT && props && 'modelValue' in props) {
+    if (
+      !isCompatEnabled(
+        DeprecationTypes.COMPONENT_V_MODEL,
+        // this is a special case where we want to use the vnode component's
+        // compat config instead of the current rendering instance (which is the
+        // parent of the component that exposes v-model)
+        { type } as any
+      )
+    ) {
+      return
+    }
+
+    if (__DEV__ && !warnedTypes.has(type as ComponentOptions)) {
+      pushWarningContext(vnode)
+      warnDeprecation(DeprecationTypes.COMPONENT_V_MODEL, { type } as any, type)
+      popWarningContext()
+      warnedTypes.add(type as ComponentOptions)
+    }
+
+    // v3 compiled model code -> v2 compat props
+    // modelValue -> value
+    // onUpdate:modelValue -> onModelCompat:input
+    const { prop = 'value', event = 'input' } = (type as any).model || {}
+    props[prop] = props.modelValue
+    delete props.modelValue
+    // important: update dynamic props
+    if (dynamicProps) {
+      dynamicProps[dynamicProps.indexOf('modelValue')] = prop
+    }
+    props[compatModelEventPrefix + event] = props['onUpdate:modelValue']
+    delete props['onUpdate:modelValue']
+  }
+}
+
+export function compatModelEmit(
+  instance: ComponentInternalInstance,
+  event: string,
+  args: any[]
+) {
+  if (!isCompatEnabled(DeprecationTypes.COMPONENT_V_MODEL, instance)) {
+    return
+  }
+  const props = instance.vnode.props
+  const modelHandler = props && props[compatModelEventPrefix + event]
+  if (modelHandler) {
+    callWithErrorHandling(
+      modelHandler,
+      instance,
+      ErrorCodes.COMPONENT_EVENT_HANDLER,
+      args
+    )
+  }
+}
diff --git a/packages/runtime-core/src/component.ts b/packages/runtime-core/src/component.ts
index 6e2c1d53..54cf487b 100644
--- a/packages/runtime-core/src/component.ts
+++ b/packages/runtime-core/src/component.ts
@@ -47,13 +47,16 @@ import {
   NO,
   makeMap,
   isPromise,
-  ShapeFlags
+  ShapeFlags,
+  extend
 } from '@vue/shared'
 import { SuspenseBoundary } from './components/Suspense'
 import { CompilerOptions } from '@vue/compiler-core'
 import { markAttrsAccessed } from './componentRenderUtils'
 import { currentRenderingInstance } from './componentRenderContext'
 import { startMeasure, endMeasure } from './profiling'
+import { convertLegacyRenderFn } from './compat/renderFn'
+import { globalCompatConfig, validateCompatConfig } from './compat/compatConfig'
 
 export type Data = Record<string, unknown>
 
@@ -93,6 +96,10 @@ export interface ComponentInternalOptions {
    * @internal
    */
   __hmrId?: string
+  /**
+   * Compat build only, for bailing out of certain compatibility behavior
+   */
+  __isBuiltIn?: boolean
   /**
    * This one should be exposed so that devtools can make use of it
    */
@@ -185,6 +192,10 @@ export type InternalRenderFunction = {
     $options: ComponentInternalInstance['ctx']
   ): VNodeChild
   _rc?: boolean // isRuntimeCompiled
+
+  // __COMPAT__ only
+  _compatChecked?: boolean // v3 and already checked for v2 compat
+  _compatWrapped?: boolean // is wrapped for v2 compat
 }
 
 /**
@@ -257,6 +268,11 @@ export interface ComponentInternalInstance {
    * @internal
    */
   directives: Record<string, Directive> | null
+  /**
+   * Resolved filters registry, v2 compat only
+   * @internal
+   */
+  filters?: Record<string, Function>
   /**
    * resolved props options
    * @internal
@@ -562,6 +578,13 @@ function setupStatefulComponent(
         validateDirectiveName(names[i])
       }
     }
+    if (Component.compilerOptions && isRuntimeOnly()) {
+      warn(
+        `"compilerOptions" is only supported when using a build of Vue that ` +
+          `includes the runtime compiler. Since you are using a runtime-only ` +
+          `build, the options should be passed via your build tool config instead.`
+      )
+    }
   }
   // 0. create render proxy property access cache
   instance.accessCache = Object.create(null)
@@ -674,12 +697,21 @@ export function registerRuntimeCompiler(_compile: any) {
   compile = _compile
 }
 
-function finishComponentSetup(
+export function finishComponentSetup(
   instance: ComponentInternalInstance,
-  isSSR: boolean
+  isSSR: boolean,
+  skipOptions?: boolean
 ) {
   const Component = instance.type as ComponentOptions
 
+  if (__COMPAT__) {
+    convertLegacyRenderFn(instance)
+
+    if (__DEV__ && Component.compatConfig) {
+      validateCompatConfig(Component.compatConfig)
+    }
+  }
+
   // template / render function normalization
   if (__NODE_JS__ && isSSR) {
     // 1. the render function may already exist, returned by `setup`
@@ -692,16 +724,42 @@ function finishComponentSetup(
       NOOP) as InternalRenderFunction
   } else if (!instance.render) {
     // could be set from setup()
-    if (compile && Component.template && !Component.render) {
-      if (__DEV__) {
-        startMeasure(instance, `compile`)
-      }
-      Component.render = compile(Component.template, {
-        isCustomElement: instance.appContext.config.isCustomElement,
-        delimiters: Component.delimiters
-      })
-      if (__DEV__) {
-        endMeasure(instance, `compile`)
+    if (compile && !Component.render) {
+      const template =
+        (__COMPAT__ &&
+          instance.vnode.props &&
+          instance.vnode.props['inline-template']) ||
+        Component.template
+      if (template) {
+        if (__DEV__) {
+          startMeasure(instance, `compile`)
+        }
+        const { isCustomElement, compilerOptions } = instance.appContext.config
+        const {
+          delimiters,
+          compilerOptions: componentCompilerOptions
+        } = Component
+        const finalCompilerOptions: CompilerOptions = extend(
+          extend(
+            {
+              isCustomElement,
+              delimiters
+            },
+            compilerOptions
+          ),
+          componentCompilerOptions
+        )
+        if (__COMPAT__) {
+          // pass runtime compat config into the compiler
+          finalCompilerOptions.compatConfig = Object.create(globalCompatConfig)
+          if (Component.compatConfig) {
+            extend(finalCompilerOptions.compatConfig, Component.compatConfig)
+          }
+        }
+        Component.render = compile(template, finalCompilerOptions)
+        if (__DEV__) {
+          endMeasure(instance, `compile`)
+        }
       }
     }
 
@@ -719,7 +777,7 @@ function finishComponentSetup(
   }
 
   // support for 2.x options
-  if (__FEATURE_OPTIONS_API__) {
+  if (__FEATURE_OPTIONS_API__ && !(__COMPAT__ && skipOptions)) {
     currentInstance = instance
     pauseTracking()
     applyOptions(instance, Component)
diff --git a/packages/runtime-core/src/componentEmits.ts b/packages/runtime-core/src/componentEmits.ts
index ab382edc..d5e2cb9f 100644
--- a/packages/runtime-core/src/componentEmits.ts
+++ b/packages/runtime-core/src/componentEmits.ts
@@ -21,6 +21,8 @@ import { warn } from './warning'
 import { UnionToIntersection } from './helpers/typeUtils'
 import { devtoolsComponentEmit } from './devtools'
 import { AppContext } from './apiCreateApp'
+import { emit as compatInstanceEmit } from './compat/instanceEventEmitter'
+import { compatModelEventPrefix, compatModelEmit } from './compat/vModel'
 
 export type ObjectEmitsOptions = Record<
   string,
@@ -56,7 +58,14 @@ export function emit(
       propsOptions: [propsOptions]
     } = instance
     if (emitsOptions) {
-      if (!(event in emitsOptions)) {
+      if (
+        !(event in emitsOptions) &&
+        !(
+          __COMPAT__ &&
+          (event.startsWith('hook:') ||
+            event.startsWith(compatModelEventPrefix))
+        )
+      ) {
         if (!propsOptions || !(toHandlerKey(event) in propsOptions)) {
           warn(
             `Component emitted event "${event}" but it is neither declared in ` +
@@ -148,6 +157,11 @@ export function emit(
       args
     )
   }
+
+  if (__COMPAT__) {
+    compatModelEmit(instance, event, args)
+    return compatInstanceEmit(instance, event, args)
+  }
 }
 
 export function normalizeEmitsOptions(
@@ -205,6 +219,11 @@ export function isEmitListener(
   if (!options || !isOn(key)) {
     return false
   }
+
+  if (__COMPAT__ && key.startsWith(compatModelEventPrefix)) {
+    return true
+  }
+
   key = key.slice(2).replace(/Once$/, '')
   return (
     hasOwn(options, key[0].toLowerCase() + key.slice(1)) ||
diff --git a/packages/runtime-core/src/componentOptions.ts b/packages/runtime-core/src/componentOptions.ts
index 7b4081af..366ac379 100644
--- a/packages/runtime-core/src/componentOptions.ts
+++ b/packages/runtime-core/src/componentOptions.ts
@@ -65,6 +65,19 @@ import { warn } from './warning'
 import { VNodeChild } from './vnode'
 import { callWithAsyncErrorHandling } from './errorHandling'
 import { UnionToIntersection } from './helpers/typeUtils'
+import { deepMergeData } from './compat/data'
+import { DeprecationTypes } from './compat/compatConfig'
+import {
+  CompatConfig,
+  isCompatEnabled,
+  softAssertCompatEnabled
+} from './compat/compatConfig'
+import {
+  AssetTypes,
+  COMPONENTS,
+  DIRECTIVES,
+  FILTERS
+} from './helpers/resolveAssets'
 
 /**
  * Interface for declaring custom options.
@@ -137,6 +150,9 @@ export interface ComponentOptionsBase<
   expose?: string[]
   serverPrefetch?(): Promise<any>
 
+  // Runtime compiler only -----------------------------------------------------
+  compilerOptions?: RuntimeCompilerOptions
+
   // Internal ------------------------------------------------------------------
 
   /**
@@ -190,6 +206,16 @@ export interface ComponentOptionsBase<
   __defaults?: Defaults
 }
 
+/**
+ * Subset of compiler options that makes sense for the runtime.
+ */
+export interface RuntimeCompilerOptions {
+  isCustomElement?: (tag: string) => boolean
+  whitespace?: 'preserve' | 'condense'
+  comments?: boolean
+  delimiters?: [string, string]
+}
+
 export type ComponentOptionsWithoutProps<
   Props = {},
   RawBindings = {},
@@ -347,10 +373,11 @@ export type ExtractComputedReturns<T extends any> = {
     : T[key] extends (...args: any[]) => infer TReturn ? TReturn : never
 }
 
-type WatchOptionItem =
-  | string
-  | WatchCallback
-  | { handler: WatchCallback | string } & WatchOptions
+export type ObjectWatchOptionItem = {
+  handler: WatchCallback | string
+} & WatchOptions
+
+type WatchOptionItem = string | WatchCallback | ObjectWatchOptionItem
 
 type ComponentWatchOptionItem = WatchOptionItem | WatchOptionItem[]
 
@@ -371,6 +398,8 @@ interface LegacyOptions<
   Mixin extends ComponentOptionsMixin,
   Extends extends ComponentOptionsMixin
 > {
+  compatConfig?: CompatConfig
+
   // allow any custom options
   [key: string]: any
 
@@ -404,6 +433,9 @@ interface LegacyOptions<
   provide?: Data | Function
   inject?: ComponentInjectOptions
 
+  // assets
+  filters?: Record<string, Function>
+
   // composition
   mixins?: Mixin[]
   extends?: Extends
@@ -427,7 +459,10 @@ interface LegacyOptions<
   renderTriggered?: DebuggerHook
   errorCaptured?: ErrorCapturedHook
 
-  // runtime compile only
+  /**
+   * runtime compile only
+   * @deprecated use `compilerOptions.delimiters` instead.
+   */
   delimiters?: [string, string]
 
   /**
@@ -490,6 +525,10 @@ export function applyOptions(
   deferredProvide: (Data | Function)[] = [],
   asMixin: boolean = false
 ) {
+  if (__COMPAT__ && isFunction(options)) {
+    options = options.options
+  }
+
   const {
     // composition
     mixins,
@@ -501,9 +540,6 @@ export function applyOptions(
     watch: watchOptions,
     provide: provideOptions,
     inject: injectOptions,
-    // assets
-    components,
-    directives,
     // lifecycle
     beforeMount,
     mounted,
@@ -588,31 +624,7 @@ export function applyOptions(
   // - watch (deferred since it relies on `this` access)
 
   if (injectOptions) {
-    if (isArray(injectOptions)) {
-      for (let i = 0; i < injectOptions.length; i++) {
-        const key = injectOptions[i]
-        ctx[key] = inject(key)
-        if (__DEV__) {
-          checkDuplicateProperties!(OptionTypes.INJECT, key)
-        }
-      }
-    } else {
-      for (const key in injectOptions) {
-        const opt = injectOptions[key]
-        if (isObject(opt)) {
-          ctx[key] = inject(
-            opt.from || key,
-            opt.default,
-            true /* treat default function as factory */
-          )
-        } else {
-          ctx[key] = inject(opt)
-        }
-        if (__DEV__) {
-          checkDuplicateProperties!(OptionTypes.INJECT, key)
-        }
-      }
-    }
+    resolveInjections(injectOptions, ctx, checkDuplicateProperties)
   }
 
   if (methods) {
@@ -736,25 +748,10 @@ export function applyOptions(
   // To reduce memory usage, only components with mixins or extends will have
   // resolved asset registry attached to instance.
   if (asMixin) {
-    if (components) {
-      extend(
-        instance.components ||
-          (instance.components = extend(
-            {},
-            (instance.type as ComponentOptions).components
-          ) as Record<string, ConcreteComponent>),
-        components
-      )
-    }
-    if (directives) {
-      extend(
-        instance.directives ||
-          (instance.directives = extend(
-            {},
-            (instance.type as ComponentOptions).directives
-          )),
-        directives
-      )
+    resolveInstanceAssets(instance, options, COMPONENTS)
+    resolveInstanceAssets(instance, options, DIRECTIVES)
+    if (__COMPAT__ && isCompatEnabled(DeprecationTypes.FILTERS, instance)) {
+      resolveInstanceAssets(instance, options, FILTERS)
     }
   }
 
@@ -795,19 +792,28 @@ export function applyOptions(
   if (renderTriggered) {
     onRenderTriggered(renderTriggered.bind(publicThis))
   }
-  if (__DEV__ && beforeDestroy) {
-    warn(`\`beforeDestroy\` has been renamed to \`beforeUnmount\`.`)
-  }
   if (beforeUnmount) {
     onBeforeUnmount(beforeUnmount.bind(publicThis))
   }
-  if (__DEV__ && destroyed) {
-    warn(`\`destroyed\` has been renamed to \`unmounted\`.`)
-  }
   if (unmounted) {
     onUnmounted(unmounted.bind(publicThis))
   }
 
+  if (__COMPAT__) {
+    if (
+      beforeDestroy &&
+      softAssertCompatEnabled(DeprecationTypes.OPTIONS_BEFORE_DESTROY, instance)
+    ) {
+      onBeforeUnmount(beforeDestroy.bind(publicThis))
+    }
+    if (
+      destroyed &&
+      softAssertCompatEnabled(DeprecationTypes.OPTIONS_DESTROYED, instance)
+    ) {
+      onUnmounted(destroyed.bind(publicThis))
+    }
+  }
+
   if (isArray(expose)) {
     if (!asMixin) {
       if (expose.length) {
@@ -824,6 +830,55 @@ export function applyOptions(
   }
 }
 
+function resolveInstanceAssets(
+  instance: ComponentInternalInstance,
+  mixin: ComponentOptions,
+  type: AssetTypes
+) {
+  if (mixin[type]) {
+    extend(
+      instance[type] ||
+        (instance[type] = extend(
+          {},
+          (instance.type as ComponentOptions)[type]
+        ) as any),
+      mixin[type]
+    )
+  }
+}
+
+export function resolveInjections(
+  injectOptions: ComponentInjectOptions,
+  ctx: any,
+  checkDuplicateProperties = NOOP as any
+) {
+  if (isArray(injectOptions)) {
+    for (let i = 0; i < injectOptions.length; i++) {
+      const key = injectOptions[i]
+      ctx[key] = inject(key)
+      if (__DEV__) {
+        checkDuplicateProperties!(OptionTypes.INJECT, key)
+      }
+    }
+  } else {
+    for (const key in injectOptions) {
+      const opt = injectOptions[key]
+      if (isObject(opt)) {
+        ctx[key] = inject(
+          opt.from || key,
+          opt.default,
+          true /* treat default function as factory */
+        )
+      } else {
+        ctx[key] = inject(opt)
+      }
+      if (__DEV__) {
+        checkDuplicateProperties!(OptionTypes.INJECT, key)
+      }
+    }
+  }
+}
+
 function callSyncHook(
   name: 'beforeCreate' | 'created',
   type: LifecycleHooks,
@@ -854,7 +909,13 @@ function callHookWithMixinAndExtends(
     }
   }
   if (selfHook) {
-    callWithAsyncErrorHandling(selfHook.bind(instance.proxy!), instance, type)
+    callWithAsyncErrorHandling(
+      __COMPAT__ && isArray(selfHook)
+        ? selfHook.map(h => h.bind(instance.proxy!))
+        : selfHook.bind(instance.proxy!),
+      instance,
+      type
+    )
   }
 }
 
@@ -904,11 +965,18 @@ function resolveData(
     instance.data = reactive(data)
   } else {
     // existing data: this is a mixin or extends.
-    extend(instance.data, data)
+    if (
+      __COMPAT__ &&
+      isCompatEnabled(DeprecationTypes.OPTIONS_DATA_MERGE, instance)
+    ) {
+      deepMergeData(instance.data, data, instance)
+    } else {
+      extend(instance.data, data)
+    }
   }
 }
 
-function createWatcher(
+export function createWatcher(
   raw: ComponentWatchOptionItem,
   ctx: Data,
   publicThis: ComponentPublicInstance,
@@ -958,19 +1026,30 @@ export function resolveMergedOptions(
   return (raw.__merged = options)
 }
 
-function mergeOptions(to: any, from: any, instance: ComponentInternalInstance) {
-  const strats = instance.appContext.config.optionMergeStrategies
+export function mergeOptions(
+  to: any,
+  from: any,
+  instance?: ComponentInternalInstance | null,
+  strats = instance && instance.appContext.config.optionMergeStrategies
+) {
+  if (__COMPAT__ && isFunction(from)) {
+    from = from.options
+  }
+
   const { mixins, extends: extendsOptions } = from
 
-  extendsOptions && mergeOptions(to, extendsOptions, instance)
+  extendsOptions && mergeOptions(to, extendsOptions, instance, strats)
   mixins &&
-    mixins.forEach((m: ComponentOptionsMixin) => mergeOptions(to, m, instance))
+    mixins.forEach((m: ComponentOptionsMixin) =>
+      mergeOptions(to, m, instance, strats)
+    )
 
   for (const key in from) {
     if (strats && hasOwn(strats, key)) {
-      to[key] = strats[key](to[key], from[key], instance.proxy, key)
+      to[key] = strats[key](to[key], from[key], instance && instance.proxy, key)
     } else {
       to[key] = from[key]
     }
   }
+  return to
 }
diff --git a/packages/runtime-core/src/componentProps.ts b/packages/runtime-core/src/componentProps.ts
index 0238bf80..5d38325d 100644
--- a/packages/runtime-core/src/componentProps.ts
+++ b/packages/runtime-core/src/componentProps.ts
@@ -33,6 +33,10 @@ import {
 import { isEmitListener } from './componentEmits'
 import { InternalObjectKey } from './vnode'
 import { AppContext } from './apiCreateApp'
+import { createPropsDefaultThis } from './compat/props'
+import { isCompatEnabled, softAssertCompatEnabled } from './compat/compatConfig'
+import { DeprecationTypes } from './compat/compatConfig'
+import { shouldSkipAttr } from './compat/attrsFallthrough'
 
 export type ComponentPropsOptions<P = Data> =
   | ComponentObjectPropsOptions<P>
@@ -184,6 +188,7 @@ export function updateProps(
   } = instance
   const rawCurrentProps = toRaw(props)
   const [options] = instance.propsOptions
+  let hasAttrsChanged = false
 
   if (
     // always force full diff in dev
@@ -209,7 +214,10 @@ export function updateProps(
           // attr / props separation was done on init and will be consistent
           // in this code path, so just check if attrs have it.
           if (hasOwn(attrs, key)) {
-            attrs[key] = value
+            if (value !== attrs[key]) {
+              attrs[key] = value
+              hasAttrsChanged = true
+            }
           } else {
             const camelizedKey = camelize(key)
             props[camelizedKey] = resolvePropValue(
@@ -221,13 +229,21 @@ export function updateProps(
             )
           }
         } else {
-          attrs[key] = value
+          if (__COMPAT__ && shouldSkipAttr(key, attrs[key], instance)) {
+            continue
+          }
+          if (value !== attrs[key]) {
+            attrs[key] = value
+            hasAttrsChanged = true
+          }
         }
       }
     }
   } else {
     // full props update.
-    setFullProps(instance, rawProps, props, attrs)
+    if (setFullProps(instance, rawProps, props, attrs)) {
+      hasAttrsChanged = true
+    }
     // in case of dynamic props, check if we need to delete keys from
     // the props object
     let kebabKey: string
@@ -267,13 +283,16 @@ export function updateProps(
       for (const key in attrs) {
         if (!rawProps || !hasOwn(rawProps, key)) {
           delete attrs[key]
+          hasAttrsChanged = true
         }
       }
     }
   }
 
   // trigger updates for $attrs in case it's used in component slots
-  trigger(instance, TriggerOpTypes.SET, '$attrs')
+  if (hasAttrsChanged) {
+    trigger(instance, TriggerOpTypes.SET, '$attrs')
+  }
 
   if (__DEV__) {
     validateProps(rawProps || {}, props, instance)
@@ -287,12 +306,27 @@ function setFullProps(
   attrs: Data
 ) {
   const [options, needCastKeys] = instance.propsOptions
+  let hasAttrsChanged = false
   if (rawProps) {
     for (const key in rawProps) {
       // key, ref are reserved and never passed down
       if (isReservedProp(key)) {
         continue
       }
+
+      if (__COMPAT__) {
+        if (key.startsWith('onHook:')) {
+          softAssertCompatEnabled(
+            DeprecationTypes.INSTANCE_EVENT_HOOKS,
+            instance,
+            key.slice(2).toLowerCase()
+          )
+        }
+        if (key === 'inline-template') {
+          continue
+        }
+      }
+
       const value = rawProps[key]
       // prop option names are camelized during normalization, so to support
       // kebab -> camel conversion here we need to camelize the key.
@@ -303,7 +337,13 @@ function setFullProps(
         // Any non-declared (either as a prop or an emitted event) props are put
         // into a separate `attrs` object for spreading. Make sure to preserve
         // original key casing
-        attrs[key] = value
+        if (__COMPAT__ && shouldSkipAttr(key, attrs[key], instance)) {
+          continue
+        }
+        if (value !== attrs[key]) {
+          attrs[key] = value
+          hasAttrsChanged = true
+        }
       }
     }
   }
@@ -321,6 +361,8 @@ function setFullProps(
       )
     }
   }
+
+  return hasAttrsChanged
 }
 
 function resolvePropValue(
@@ -342,7 +384,13 @@ function resolvePropValue(
           value = propsDefaults[key]
         } else {
           setCurrentInstance(instance)
-          value = propsDefaults[key] = defaultValue(props)
+          value = propsDefaults[key] = defaultValue.call(
+            __COMPAT__ &&
+            isCompatEnabled(DeprecationTypes.PROPS_DEFAULT_THIS, instance)
+              ? createPropsDefaultThis(instance, props, key)
+              : null,
+            props
+          )
           setCurrentInstance(null)
         }
       } else {
@@ -381,6 +429,9 @@ export function normalizePropsOptions(
   let hasExtends = false
   if (__FEATURE_OPTIONS_API__ && !isFunction(comp)) {
     const extendProps = (raw: ComponentOptions) => {
+      if (__COMPAT__ && isFunction(raw)) {
+        raw = raw.options
+      }
       hasExtends = true
       const [props, keys] = normalizePropsOptions(raw, appContext, true)
       extend(normalized, props)
diff --git a/packages/runtime-core/src/componentPublicInstance.ts b/packages/runtime-core/src/componentPublicInstance.ts
index fd0351f6..9610aad1 100644
--- a/packages/runtime-core/src/componentPublicInstance.ts
+++ b/packages/runtime-core/src/componentPublicInstance.ts
@@ -11,7 +11,8 @@ import {
   isGloballyWhitelisted,
   NOOP,
   extend,
-  isString
+  isString,
+  isFunction
 } from '@vue/shared'
 import {
   ReactiveEffect,
@@ -40,6 +41,7 @@ import { markAttrsAccessed } from './componentRenderUtils'
 import { currentRenderingInstance } from './componentRenderContext'
 import { warn } from './warning'
 import { UnionToIntersection } from './helpers/typeUtils'
+import { installCompatInstanceProperties } from './compat/instance'
 
 /**
  * Custom properties added to component instances in any way and can be accessed through `this`
@@ -201,7 +203,10 @@ export type ComponentPublicInstance<
   M &
   ComponentCustomProperties
 
-type PublicPropertiesMap = Record<string, (i: ComponentInternalInstance) => any>
+export type PublicPropertiesMap = Record<
+  string,
+  (i: ComponentInternalInstance) => any
+>
 
 /**
  * #2437 In Vue 3, functional components do not have a public instance proxy but
@@ -233,6 +238,10 @@ const publicPropertiesMap: PublicPropertiesMap = extend(Object.create(null), {
   $watch: i => (__FEATURE_OPTIONS_API__ ? instanceWatch.bind(i) : NOOP)
 } as PublicPropertiesMap)
 
+if (__COMPAT__) {
+  installCompatInstanceProperties(publicPropertiesMap)
+}
+
 const enum AccessTypes {
   SETUP,
   DATA,
@@ -335,7 +344,17 @@ export const PublicInstanceProxyHandlers: ProxyHandler<any> = {
       ((globalProperties = appContext.config.globalProperties),
       hasOwn(globalProperties, key))
     ) {
-      return globalProperties[key]
+      if (__COMPAT__) {
+        const desc = Object.getOwnPropertyDescriptor(globalProperties, key)!
+        if (desc.get) {
+          return desc.get.call(instance.proxy)
+        } else {
+          const val = globalProperties[key]
+          return isFunction(val) ? val.bind(instance.proxy) : val
+        }
+      } else {
+        return globalProperties[key]
+      }
     } else if (
       __DEV__ &&
       currentRenderingInstance &&
@@ -483,17 +502,6 @@ export function createRenderContext(instance: ComponentInternalInstance) {
     })
   })
 
-  // expose global properties
-  const { globalProperties } = instance.appContext.config
-  Object.keys(globalProperties).forEach(key => {
-    Object.defineProperty(target, key, {
-      configurable: true,
-      enumerable: false,
-      get: () => globalProperties[key],
-      set: NOOP
-    })
-  })
-
   return target as ComponentRenderContext
 }
 
diff --git a/packages/runtime-core/src/componentRenderContext.ts b/packages/runtime-core/src/componentRenderContext.ts
index 1771d9aa..6cc23235 100644
--- a/packages/runtime-core/src/componentRenderContext.ts
+++ b/packages/runtime-core/src/componentRenderContext.ts
@@ -25,6 +25,10 @@ export function setCurrentRenderingInstance(
   const prev = currentRenderingInstance
   currentRenderingInstance = instance
   currentScopeId = (instance && instance.type.__scopeId) || null
+  // v2 pre-compiled components uses _scopeId instead of __scopeId
+  if (__COMPAT__ && !currentScopeId) {
+    currentScopeId = (instance && (instance.type as any)._scopeId) || null
+  }
   return prev
 }
 
diff --git a/packages/runtime-core/src/componentRenderUtils.ts b/packages/runtime-core/src/componentRenderUtils.ts
index 1274f474..ca215e54 100644
--- a/packages/runtime-core/src/componentRenderUtils.ts
+++ b/packages/runtime-core/src/componentRenderUtils.ts
@@ -1,7 +1,8 @@
 import {
   ComponentInternalInstance,
   FunctionalComponent,
-  Data
+  Data,
+  getComponentName
 } from './component'
 import {
   VNode,
@@ -20,6 +21,11 @@ import { isHmrUpdating } from './hmr'
 import { NormalizedProps } from './componentProps'
 import { isEmitListener } from './componentEmits'
 import { setCurrentRenderingInstance } from './componentRenderContext'
+import {
+  DeprecationTypes,
+  isCompatEnabled,
+  warnDeprecation
+} from './compat/compatConfig'
 
 /**
  * dev only flag to track whether $attrs was used during render.
@@ -117,7 +123,7 @@ export function renderComponentRoot(
       ;[root, setRoot] = getChildRoot(result)
     }
 
-    if (Component.inheritAttrs !== false && fallthroughAttrs) {
+    if (fallthroughAttrs && Component.inheritAttrs !== false) {
       const keys = Object.keys(fallthroughAttrs)
       const { shapeFlag } = root
       if (keys.length) {
@@ -175,6 +181,29 @@ export function renderComponentRoot(
       }
     }
 
+    if (
+      __COMPAT__ &&
+      isCompatEnabled(DeprecationTypes.INSTANCE_ATTRS_CLASS_STYLE, instance) &&
+      vnode.shapeFlag & ShapeFlags.STATEFUL_COMPONENT &&
+      (root.shapeFlag & ShapeFlags.ELEMENT ||
+        root.shapeFlag & ShapeFlags.COMPONENT)
+    ) {
+      const { class: cls, style } = vnode.props || {}
+      if (cls || style) {
+        if (__DEV__ && Component.inheritAttrs === false) {
+          warnDeprecation(
+            DeprecationTypes.INSTANCE_ATTRS_CLASS_STYLE,
+            instance,
+            getComponentName(instance.type)
+          )
+        }
+        root = cloneVNode(root, {
+          class: cls,
+          style: style
+        })
+      }
+    }
+
     // inherit directives
     if (vnode.dirs) {
       if (__DEV__ && !isElementRoot(root)) {
diff --git a/packages/runtime-core/src/componentSlots.ts b/packages/runtime-core/src/componentSlots.ts
index f1eacd87..9fdb0d14 100644
--- a/packages/runtime-core/src/componentSlots.ts
+++ b/packages/runtime-core/src/componentSlots.ts
@@ -19,6 +19,7 @@ import { warn } from './warning'
 import { isKeepAlive } from './components/KeepAlive'
 import { withCtx } from './componentRenderContext'
 import { isHmrUpdating } from './hmr'
+import { DeprecationTypes, isCompatEnabled } from './compat/compatConfig'
 
 export type Slot = (...args: any[]) => VNode[]
 
@@ -72,7 +73,11 @@ const normalizeSlot = (
     return normalizeSlotValue(rawSlot(props))
   }, ctx) as Slot
 
-const normalizeObjectSlots = (rawSlots: RawSlots, slots: InternalSlots) => {
+const normalizeObjectSlots = (
+  rawSlots: RawSlots,
+  slots: InternalSlots,
+  instance: ComponentInternalInstance
+) => {
   const ctx = rawSlots._ctx
   for (const key in rawSlots) {
     if (isInternalKey(key)) continue
@@ -80,7 +85,13 @@ const normalizeObjectSlots = (rawSlots: RawSlots, slots: InternalSlots) => {
     if (isFunction(value)) {
       slots[key] = normalizeSlot(key, value, ctx)
     } else if (value != null) {
-      if (__DEV__) {
+      if (
+        __DEV__ &&
+        !(
+          __COMPAT__ &&
+          isCompatEnabled(DeprecationTypes.RENDER_FUNCTION, instance)
+        )
+      ) {
         warn(
           `Non-function value encountered for slot "${key}". ` +
             `Prefer function slots for better performance.`
@@ -96,7 +107,11 @@ const normalizeVNodeSlots = (
   instance: ComponentInternalInstance,
   children: VNodeNormalizedChildren
 ) => {
-  if (__DEV__ && !isKeepAlive(instance.vnode)) {
+  if (
+    __DEV__ &&
+    !isKeepAlive(instance.vnode) &&
+    !(__COMPAT__ && isCompatEnabled(DeprecationTypes.RENDER_FUNCTION, instance))
+  ) {
     warn(
       `Non-function value encountered for default slot. ` +
         `Prefer function slots for better performance.`
@@ -117,7 +132,11 @@ export const initSlots = (
       // make compiler marker non-enumerable
       def(children as InternalSlots, '_', type)
     } else {
-      normalizeObjectSlots(children as RawSlots, (instance.slots = {}))
+      normalizeObjectSlots(
+        children as RawSlots,
+        (instance.slots = {}),
+        instance
+      )
     }
   } else {
     instance.slots = {}
@@ -162,7 +181,7 @@ export const updateSlots = (
       }
     } else {
       needDeletionCheck = !(children as RawSlots).$stable
-      normalizeObjectSlots(children as RawSlots, slots)
+      normalizeObjectSlots(children as RawSlots, slots, instance)
     }
     deletionComparisonTarget = children as RawSlots
   } else if (children) {
diff --git a/packages/runtime-core/src/components/BaseTransition.ts b/packages/runtime-core/src/components/BaseTransition.ts
index a6f2fb27..1cb47c81 100644
--- a/packages/runtime-core/src/components/BaseTransition.ts
+++ b/packages/runtime-core/src/components/BaseTransition.ts
@@ -1,7 +1,8 @@
 import {
   getCurrentInstance,
   SetupContext,
-  ComponentInternalInstance
+  ComponentInternalInstance,
+  ComponentOptions
 } from '../component'
 import {
   cloneVNode,
@@ -110,7 +111,7 @@ export function useTransitionState(): TransitionState {
 
 const TransitionHookValidator = [Function, Array]
 
-const BaseTransitionImpl = {
+const BaseTransitionImpl: ComponentOptions = {
   name: `BaseTransition`,
 
   props: {
@@ -250,6 +251,10 @@ const BaseTransitionImpl = {
   }
 }
 
+if (__COMPAT__) {
+  BaseTransitionImpl.__isBuiltIn = true
+}
+
 // export the public type for h/tsx inference
 // also to avoid inline import() in generated d.ts files
 export const BaseTransition = (BaseTransitionImpl as any) as {
diff --git a/packages/runtime-core/src/components/KeepAlive.ts b/packages/runtime-core/src/components/KeepAlive.ts
index 37836c58..9226b775 100644
--- a/packages/runtime-core/src/components/KeepAlive.ts
+++ b/packages/runtime-core/src/components/KeepAlive.ts
@@ -5,7 +5,8 @@ import {
   ComponentInternalInstance,
   LifecycleHooks,
   currentInstance,
-  getComponentName
+  getComponentName,
+  ComponentOptions
 } from '../component'
 import { VNode, cloneVNode, isVNode, VNodeProps } from '../vnode'
 import { warn } from '../warning'
@@ -63,7 +64,7 @@ export interface KeepAliveContext extends ComponentRenderContext {
 export const isKeepAlive = (vnode: VNode): boolean =>
   (vnode.type as any).__isKeepAlive
 
-const KeepAliveImpl = {
+const KeepAliveImpl: ComponentOptions = {
   name: `KeepAlive`,
 
   // Marker for special handling inside the renderer. We are not using a ===
@@ -313,6 +314,10 @@ const KeepAliveImpl = {
   }
 }
 
+if (__COMPAT__) {
+  KeepAliveImpl.__isBuildIn = true
+}
+
 // export the public type for h/tsx inference
 // also to avoid inline import() in generated d.ts files
 export const KeepAlive = (KeepAliveImpl as any) as {
diff --git a/packages/runtime-core/src/directives.ts b/packages/runtime-core/src/directives.ts
index 20f25d03..a1987908 100644
--- a/packages/runtime-core/src/directives.ts
+++ b/packages/runtime-core/src/directives.ts
@@ -18,6 +18,7 @@ import { ComponentInternalInstance, Data } from './component'
 import { currentRenderingInstance } from './componentRenderContext'
 import { callWithAsyncErrorHandling, ErrorCodes } from './errorHandling'
 import { ComponentPublicInstance } from './componentPublicInstance'
+import { mapCompatDirectiveHook } from './compat/customDirective'
 
 export interface DirectiveBinding<V = any> {
   instance: ComponentPublicInstance | null
@@ -124,7 +125,10 @@ export function invokeDirectiveHook(
     if (oldBindings) {
       binding.oldValue = oldBindings[i].value
     }
-    const hook = binding.dir[name] as DirectiveHook | undefined
+    let hook = binding.dir[name] as DirectiveHook | DirectiveHook[] | undefined
+    if (__COMPAT__ && !hook) {
+      hook = mapCompatDirectiveHook(name, binding.dir, instance)
+    }
     if (hook) {
       callWithAsyncErrorHandling(hook, instance, ErrorCodes.DIRECTIVE_HOOK, [
         vnode.el,
diff --git a/packages/runtime-core/src/helpers/resolveAssets.ts b/packages/runtime-core/src/helpers/resolveAssets.ts
index e867cc51..119bdbe6 100644
--- a/packages/runtime-core/src/helpers/resolveAssets.ts
+++ b/packages/runtime-core/src/helpers/resolveAssets.ts
@@ -10,8 +10,11 @@ import { camelize, capitalize, isString } from '@vue/shared'
 import { warn } from '../warning'
 import { VNodeTypes } from '../vnode'
 
-const COMPONENTS = 'components'
-const DIRECTIVES = 'directives'
+export const COMPONENTS = 'components'
+export const DIRECTIVES = 'directives'
+export const FILTERS = 'filters'
+
+export type AssetTypes = typeof COMPONENTS | typeof DIRECTIVES | typeof FILTERS
 
 /**
  * @private
@@ -44,6 +47,14 @@ export function resolveDirective(name: string): Directive | undefined {
   return resolveAsset(DIRECTIVES, name)
 }
 
+/**
+ * v2 compat only
+ * @internal
+ */
+export function resolveFilter(name: string): Function | undefined {
+  return resolveAsset(FILTERS, name)
+}
+
 /**
  * @private
  * overload 1: components
@@ -60,8 +71,11 @@ function resolveAsset(
   name: string
 ): Directive | undefined
 // implementation
+// overload 3: filters (compat only)
+function resolveAsset(type: typeof FILTERS, name: string): Function | undefined
+// implementation
 function resolveAsset(
-  type: typeof COMPONENTS | typeof DIRECTIVES,
+  type: AssetTypes,
   name: string,
   warnMissing = true,
   maybeSelfReference = false
diff --git a/packages/runtime-core/src/index.ts b/packages/runtime-core/src/index.ts
index 6026d4c2..b4b63545 100644
--- a/packages/runtime-core/src/index.ts
+++ b/packages/runtime-core/src/index.ts
@@ -179,7 +179,8 @@ export {
   ComponentOptionsBase,
   RenderFunction,
   MethodOptions,
-  ComputedOptions
+  ComputedOptions,
+  RuntimeCompilerOptions
 } from './componentOptions'
 export { EmitsOptions, ObjectEmitsOptions } from './componentEmits'
 export {
@@ -279,3 +280,38 @@ const _ssrUtils = {
  * @internal
  */
 export const ssrUtils = (__NODE_JS__ ? _ssrUtils : null) as typeof _ssrUtils
+
+// 2.x COMPAT ------------------------------------------------------------------
+
+export { DeprecationTypes } from './compat/compatConfig'
+export { CompatVue } from './compat/global'
+export { LegacyConfig } from './compat/globalConfig'
+
+import { warnDeprecation } from './compat/compatConfig'
+import { createCompatVue } from './compat/global'
+import {
+  isCompatEnabled,
+  checkCompatEnabled,
+  softAssertCompatEnabled
+} from './compat/compatConfig'
+import { resolveFilter as _resolveFilter } from './helpers/resolveAssets'
+
+/**
+ * @internal only exposed in compat builds
+ */
+export const resolveFilter = __COMPAT__ ? _resolveFilter : null
+
+const _compatUtils = {
+  warnDeprecation,
+  createCompatVue,
+  isCompatEnabled,
+  checkCompatEnabled,
+  softAssertCompatEnabled
+}
+
+/**
+ * @internal only exposed in compat builds.
+ */
+export const compatUtils = (__COMPAT__
+  ? _compatUtils
+  : null) as typeof _compatUtils
diff --git a/packages/runtime-core/src/renderer.ts b/packages/runtime-core/src/renderer.ts
index 2187ce3e..4555b9fb 100644
--- a/packages/runtime-core/src/renderer.ts
+++ b/packages/runtime-core/src/renderer.ts
@@ -76,7 +76,6 @@ import {
 import { createHydrationFunctions, RootHydrateFunction } from './hydration'
 import { invokeDirectiveHook } from './directives'
 import { startMeasure, endMeasure } from './profiling'
-import { ComponentPublicInstance } from './componentPublicInstance'
 import {
   devtoolsComponentAdded,
   devtoolsComponentRemoved,
@@ -85,6 +84,9 @@ import {
 } from './devtools'
 import { initFeatureFlags } from './featureFlags'
 import { isAsyncWrapper } from './apiAsyncComponent'
+import { isCompatEnabled } from './compat/compatConfig'
+import { DeprecationTypes } from './compat/compatConfig'
+import { registerLegacyRef } from './compat/ref'
 
 export interface Renderer<HostElement = RendererElement> {
   render: RootRenderFunction<HostElement>
@@ -307,7 +309,8 @@ export const setRef = (
   rawRef: VNodeNormalizedRef,
   oldRawRef: VNodeNormalizedRef | null,
   parentSuspense: SuspenseBoundary | null,
-  vnode: VNode | null
+  vnode: VNode,
+  isUnmount = false
 ) => {
   if (isArray(rawRef)) {
     rawRef.forEach((r, i) =>
@@ -315,26 +318,25 @@ export const setRef = (
         r,
         oldRawRef && (isArray(oldRawRef) ? oldRawRef[i] : oldRawRef),
         parentSuspense,
-        vnode
+        vnode,
+        isUnmount
       )
     )
     return
   }
 
-  let value: ComponentPublicInstance | RendererNode | Record<string, any> | null
-  if (!vnode) {
-    // means unmount
-    value = null
-  } else if (isAsyncWrapper(vnode)) {
+  if (isAsyncWrapper(vnode) && !isUnmount) {
     // when mounting async components, nothing needs to be done,
     // because the template ref is forwarded to inner component
     return
-  } else if (vnode.shapeFlag & ShapeFlags.STATEFUL_COMPONENT) {
-    value = vnode.component!.exposed || vnode.component!.proxy
-  } else {
-    value = vnode.el
   }
 
+  const refValue =
+    vnode.shapeFlag & ShapeFlags.STATEFUL_COMPONENT
+      ? vnode.component!.exposed || vnode.component!.proxy
+      : vnode.el
+  const value = isUnmount ? null : refValue
+
   const { i: owner, r: ref } = rawRef
   if (__DEV__ && !owner) {
     warn(
@@ -347,7 +349,7 @@ export const setRef = (
   const refs = owner.refs === EMPTY_OBJ ? (owner.refs = {}) : owner.refs
   const setupState = owner.setupState
 
-  // unset old ref
+  // dynamic ref changed. unset old ref
   if (oldRef != null && oldRef !== ref) {
     if (isString(oldRef)) {
       refs[oldRef] = null
@@ -361,7 +363,11 @@ export const setRef = (
 
   if (isString(ref)) {
     const doSet = () => {
-      refs[ref] = value
+      if (__COMPAT__ && isCompatEnabled(DeprecationTypes.V_FOR_REF, owner)) {
+        registerLegacyRef(refs, ref, refValue, owner, rawRef.f, isUnmount)
+      } else {
+        refs[ref] = value
+      }
       if (hasOwn(setupState, ref)) {
         setupState[ref] = value
       }
@@ -440,6 +446,9 @@ function baseCreateRenderer(
   options: RendererOptions,
   createHydrationFns?: typeof createHydrationFunctions
 ): any {
+  const isHookEventCompatEnabled =
+    __COMPAT__ && isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, null)
+
   // compile-time feature flags check
   if (__ESM_BUNDLER__ && !__TEST__) {
     initFeatureFlags()
@@ -579,7 +588,7 @@ function baseCreateRenderer(
 
     // set ref
     if (ref != null && parentComponent) {
-      setRef(ref, n1 && n1.ref, parentSuspense, n2)
+      setRef(ref, n1 && n1.ref, parentSuspense, n2 || n1, !n2)
     }
   }
 
@@ -1292,11 +1301,16 @@ function baseCreateRenderer(
     isSVG,
     optimized
   ) => {
-    const instance: ComponentInternalInstance = (initialVNode.component = createComponentInstance(
-      initialVNode,
-      parentComponent,
-      parentSuspense
-    ))
+    // 2.x compat may pre-creaate the component instance before actually
+    // mounting
+    const compatMountInstance = __COMPAT__ && initialVNode.component
+    const instance: ComponentInternalInstance =
+      compatMountInstance ||
+      (initialVNode.component = createComponentInstance(
+        initialVNode,
+        parentComponent,
+        parentSuspense
+      ))
 
     if (__DEV__ && instance.type.__hmrId) {
       registerHMR(instance)
@@ -1313,12 +1327,14 @@ function baseCreateRenderer(
     }
 
     // resolve props and slots for setup context
-    if (__DEV__) {
-      startMeasure(instance, `init`)
-    }
-    setupComponent(instance)
-    if (__DEV__) {
-      endMeasure(instance, `init`)
+    if (!(__COMPAT__ && compatMountInstance)) {
+      if (__DEV__) {
+        startMeasure(instance, `init`)
+      }
+      setupComponent(instance)
+      if (__DEV__) {
+        endMeasure(instance, `init`)
+      }
     }
 
     // setup() is async. This component relies on async logic to be resolved
@@ -1410,6 +1426,9 @@ function baseCreateRenderer(
         if ((vnodeHook = props && props.onVnodeBeforeMount)) {
           invokeVNodeHook(vnodeHook, parent, initialVNode)
         }
+        if (__COMPAT__ && isHookEventCompatEnabled) {
+          instance.emit('hook:beforeMount')
+        }
 
         // render
         if (__DEV__) {
@@ -1460,19 +1479,29 @@ function baseCreateRenderer(
         // onVnodeMounted
         if ((vnodeHook = props && props.onVnodeMounted)) {
           const scopedInitialVNode = initialVNode
-          queuePostRenderEffect(() => {
-            invokeVNodeHook(vnodeHook!, parent, scopedInitialVNode)
-          }, parentSuspense)
+          queuePostRenderEffect(
+            () => invokeVNodeHook(vnodeHook!, parent, scopedInitialVNode),
+            parentSuspense
+          )
         }
+        if (__COMPAT__ && isHookEventCompatEnabled) {
+          queuePostRenderEffect(
+            () => instance.emit('hook:mounted'),
+            parentSuspense
+          )
+        }
+
         // activated hook for keep-alive roots.
         // #1742 activated hook must be accessed after first render
         // since the hook may be injected by a child keep-alive
-        const { a } = instance
-        if (
-          a &&
-          initialVNode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE
-        ) {
-          queuePostRenderEffect(a, parentSuspense)
+        if (initialVNode.shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE) {
+          instance.a && queuePostRenderEffect(instance.a, parentSuspense)
+          if (__COMPAT__ && isHookEventCompatEnabled) {
+            queuePostRenderEffect(
+              () => instance.emit('hook:activated'),
+              parentSuspense
+            )
+          }
         }
         instance.isMounted = true
 
@@ -1508,6 +1537,9 @@ function baseCreateRenderer(
         if ((vnodeHook = next.props && next.props.onVnodeBeforeUpdate)) {
           invokeVNodeHook(vnodeHook, parent, next, vnode)
         }
+        if (__COMPAT__ && isHookEventCompatEnabled) {
+          instance.emit('hook:beforeUpdate')
+        }
 
         // render
         if (__DEV__) {
@@ -1550,9 +1582,16 @@ function baseCreateRenderer(
         }
         // onVnodeUpdated
         if ((vnodeHook = next.props && next.props.onVnodeUpdated)) {
-          queuePostRenderEffect(() => {
-            invokeVNodeHook(vnodeHook!, parent, next!, vnode)
-          }, parentSuspense)
+          queuePostRenderEffect(
+            () => invokeVNodeHook(vnodeHook!, parent, next!, vnode),
+            parentSuspense
+          )
+        }
+        if (__COMPAT__ && isHookEventCompatEnabled) {
+          queuePostRenderEffect(
+            () => instance.emit('hook:updated'),
+            parentSuspense
+          )
         }
 
         if (__DEV__ || __FEATURE_PROD_DEVTOOLS__) {
@@ -1564,6 +1603,11 @@ function baseCreateRenderer(
         }
       }
     }, __DEV__ ? createDevEffectOptions(instance) : prodEffectOptions)
+
+    if (__DEV__) {
+      // @ts-ignore
+      instance.update.ownerInstance = instance
+    }
   }
 
   const updateComponentPreRender = (
@@ -2073,7 +2117,7 @@ function baseCreateRenderer(
     } = vnode
     // unset ref
     if (ref != null) {
-      setRef(ref, null, parentSuspense, null)
+      setRef(ref, null, parentSuspense, vnode, true)
     }
 
     if (shapeFlag & ShapeFlags.COMPONENT_SHOULD_KEEP_ALIVE) {
@@ -2204,10 +2248,15 @@ function baseCreateRenderer(
     }
 
     const { bum, effects, update, subTree, um } = instance
+
     // beforeUnmount hook
     if (bum) {
       invokeArrayFns(bum)
     }
+    if (__COMPAT__ && isHookEventCompatEnabled) {
+      instance.emit('hook:beforeDestroy')
+    }
+
     if (effects) {
       for (let i = 0; i < effects.length; i++) {
         stop(effects[i])
@@ -2223,6 +2272,12 @@ function baseCreateRenderer(
     if (um) {
       queuePostRenderEffect(um, parentSuspense)
     }
+    if (__COMPAT__ && isHookEventCompatEnabled) {
+      queuePostRenderEffect(
+        () => instance.emit('hook:destroyed'),
+        parentSuspense
+      )
+    }
     queuePostRenderEffect(() => {
       instance.isUnmounted = true
     }, parentSuspense)
diff --git a/packages/runtime-core/src/scheduler.ts b/packages/runtime-core/src/scheduler.ts
index 372866c8..8ab5a231 100644
--- a/packages/runtime-core/src/scheduler.ts
+++ b/packages/runtime-core/src/scheduler.ts
@@ -1,6 +1,8 @@
 import { ErrorCodes, callWithErrorHandling } from './errorHandling'
 import { isArray } from '@vue/shared'
 import { ComponentPublicInstance } from './componentPublicInstance'
+import { ComponentInternalInstance, getComponentName } from './component'
+import { warn } from './warning'
 
 export interface SchedulerJob {
   (): void
@@ -22,6 +24,7 @@ export interface SchedulerJob {
    * stabilizes (#1727).
    */
   allowRecurse?: boolean
+  ownerInstance?: ComponentInternalInstance
 }
 
 export type SchedulerCb = Function & { id?: number }
@@ -164,8 +167,11 @@ export function flushPreFlushCbs(
       preFlushIndex < activePreFlushCbs.length;
       preFlushIndex++
     ) {
-      if (__DEV__) {
+      if (
+        __DEV__ &&
         checkRecursiveUpdates(seen!, activePreFlushCbs[preFlushIndex])
+      ) {
+        continue
       }
       activePreFlushCbs[preFlushIndex]()
     }
@@ -200,8 +206,11 @@ export function flushPostFlushCbs(seen?: CountMap) {
       postFlushIndex < activePostFlushCbs.length;
       postFlushIndex++
     ) {
-      if (__DEV__) {
+      if (
+        __DEV__ &&
         checkRecursiveUpdates(seen!, activePostFlushCbs[postFlushIndex])
+      ) {
+        continue
       }
       activePostFlushCbs[postFlushIndex]()
     }
@@ -235,8 +244,8 @@ function flushJobs(seen?: CountMap) {
     for (flushIndex = 0; flushIndex < queue.length; flushIndex++) {
       const job = queue[flushIndex]
       if (job) {
-        if (__DEV__) {
-          checkRecursiveUpdates(seen!, job)
+        if (__DEV__ && checkRecursiveUpdates(seen!, job)) {
+          continue
         }
         callWithErrorHandling(job, null, ErrorCodes.SCHEDULER)
       }
@@ -263,13 +272,18 @@ function checkRecursiveUpdates(seen: CountMap, fn: SchedulerJob | SchedulerCb) {
   } else {
     const count = seen.get(fn)!
     if (count > RECURSION_LIMIT) {
-      throw new Error(
-        `Maximum recursive updates exceeded. ` +
+      const instance = (fn as SchedulerJob).ownerInstance
+      const componentName = instance && getComponentName(instance.type)
+      warn(
+        `Maximum recursive updates exceeded${
+          componentName ? ` in component <${componentName}>` : ``
+        }. ` +
           `This means you have a reactive effect that is mutating its own ` +
           `dependencies and thus recursively triggering itself. Possible sources ` +
           `include component template, render function, updated hook or ` +
           `watcher source function.`
       )
+      return true
     } else {
       seen.set(fn, count + 1)
     }
diff --git a/packages/runtime-core/src/vnode.ts b/packages/runtime-core/src/vnode.ts
index e46c9842..dc002225 100644
--- a/packages/runtime-core/src/vnode.ts
+++ b/packages/runtime-core/src/vnode.ts
@@ -41,6 +41,10 @@ import { RendererNode, RendererElement } from './renderer'
 import { NULL_DYNAMIC_COMPONENT } from './helpers/resolveAssets'
 import { hmrDirtyComponents } from './hmr'
 import { setCompiledSlotRendering } from './helpers/renderSlot'
+import { convertLegacyComponent } from './compat/component'
+import { convertLegacyVModelProps } from './compat/vModel'
+import { defineLegacyVNodeProperties } from './compat/renderFn'
+import { convertLegacyRefInFor } from './compat/ref'
 
 export const Fragment = (Symbol(__DEV__ ? 'Fragment' : undefined) as any) as {
   __isFragment: true
@@ -71,6 +75,7 @@ export type VNodeRef =
 export type VNodeNormalizedRefAtom = {
   i: ComponentInternalInstance
   r: VNodeRef
+  f?: boolean // v2 compat only, refInFor marker
 }
 
 export type VNodeNormalizedRef =
@@ -127,10 +132,12 @@ export interface VNode<
    * @internal
    */
   __v_isVNode: true
+
   /**
    * @internal
    */
   [ReactiveFlags.SKIP]: true
+
   type: VNodeTypes
   props: (VNodeProps & ExtraProps) | null
   key: string | number | null
@@ -358,6 +365,11 @@ function _createVNode(
     type = type.__vccOpts
   }
 
+  // 2.x async/functional component compat
+  if (__COMPAT__) {
+    type = convertLegacyComponent(type, currentRenderingInstance)
+  }
+
   // class & style normalization.
   if (props) {
     // for reactive or proxy objects, we need to clone it to enable mutation.
@@ -405,7 +417,7 @@ function _createVNode(
 
   const vnode: VNode = {
     __v_isVNode: true,
-    [ReactiveFlags.SKIP]: true,
+    __v_skip: true,
     type,
     props,
     key: props && normalizeKey(props),
@@ -463,6 +475,12 @@ function _createVNode(
     currentBlock.push(vnode)
   }
 
+  if (__COMPAT__) {
+    convertLegacyVModelProps(vnode)
+    convertLegacyRefInFor(vnode)
+    defineLegacyVNodeProperties(vnode)
+  }
+
   return vnode
 }
 
@@ -475,9 +493,9 @@ export function cloneVNode<T, U>(
   // key enumeration cost.
   const { props, ref, patchFlag, children } = vnode
   const mergedProps = extraProps ? mergeProps(props || {}, extraProps) : props
-  return {
+  const cloned: VNode = {
     __v_isVNode: true,
-    [ReactiveFlags.SKIP]: true,
+    __v_skip: true,
     type: vnode.type,
     props: mergedProps,
     key: mergedProps && normalizeKey(mergedProps),
@@ -529,6 +547,10 @@ export function cloneVNode<T, U>(
     el: vnode.el,
     anchor: vnode.anchor
   }
+  if (__COMPAT__) {
+    defineLegacyVNodeProperties(cloned)
+  }
+  return cloned as any
 }
 
 /**
@@ -671,7 +693,7 @@ export function mergeProps(...args: (Data & VNodeProps)[]) {
         const incoming = toMerge[key]
         if (existing !== incoming) {
           ret[key] = existing
-            ? [].concat(existing as any, toMerge[key] as any)
+            ? [].concat(existing as any, incoming as any)
             : incoming
         }
       } else if (key !== '') {
diff --git a/packages/runtime-core/src/warning.ts b/packages/runtime-core/src/warning.ts
index ad58778e..ad31bc81 100644
--- a/packages/runtime-core/src/warning.ts
+++ b/packages/runtime-core/src/warning.ts
@@ -71,7 +71,7 @@ export function warn(msg: string, ...args: any[]) {
   resetTracking()
 }
 
-function getComponentTrace(): ComponentTraceStack {
+export function getComponentTrace(): ComponentTraceStack {
   let currentVNode: VNode | null = stack[stack.length - 1]
   if (!currentVNode) {
     return []
diff --git a/packages/runtime-dom/src/components/Transition.ts b/packages/runtime-dom/src/components/Transition.ts
index 2c024882..a253f51b 100644
--- a/packages/runtime-dom/src/components/Transition.ts
+++ b/packages/runtime-dom/src/components/Transition.ts
@@ -3,7 +3,9 @@ import {
   BaseTransitionProps,
   h,
   warn,
-  FunctionalComponent
+  FunctionalComponent,
+  compatUtils,
+  DeprecationTypes
 } from '@vue/runtime-core'
 import { isObject, toNumber, extend } from '@vue/shared'
 
@@ -44,6 +46,10 @@ export const Transition: FunctionalComponent<TransitionProps> = (
 
 Transition.displayName = 'Transition'
 
+if (__COMPAT__) {
+  Transition.__isBuiltIn = true
+}
+
 const DOMTransitionPropsValidators = {
   name: String,
   type: String,
@@ -72,10 +78,20 @@ export const TransitionPropsValidators = (Transition.props = /*#__PURE__*/ exten
 export function resolveTransitionProps(
   rawProps: TransitionProps
 ): BaseTransitionProps<Element> {
-  let {
+  const baseProps: BaseTransitionProps<Element> = {}
+  for (const key in rawProps) {
+    if (!(key in DOMTransitionPropsValidators)) {
+      ;(baseProps as any)[key] = (rawProps as any)[key]
+    }
+  }
+
+  if (rawProps.css === false) {
+    return baseProps
+  }
+
+  const {
     name = 'v',
     type,
-    css = true,
     duration,
     enterFromClass = `${name}-enter-from`,
     enterActiveClass = `${name}-enter-active`,
@@ -88,15 +104,24 @@ export function resolveTransitionProps(
     leaveToClass = `${name}-leave-to`
   } = rawProps
 
-  const baseProps: BaseTransitionProps<Element> = {}
-  for (const key in rawProps) {
-    if (!(key in DOMTransitionPropsValidators)) {
-      ;(baseProps as any)[key] = (rawProps as any)[key]
+  // legacy transition class compat
+  const legacyClassEnabled =
+    __COMPAT__ &&
+    compatUtils.isCompatEnabled(DeprecationTypes.TRANSITION_CLASSES, null)
+  let legacyEnterFromClass: string
+  let legacyAppearFromClass: string
+  let legacyLeaveFromClass: string
+  if (__COMPAT__ && legacyClassEnabled) {
+    const toLegacyClass = (cls: string) => cls.replace(/-from$/, '')
+    if (!rawProps.enterFromClass) {
+      legacyEnterFromClass = toLegacyClass(enterFromClass)
+    }
+    if (!rawProps.appearFromClass) {
+      legacyAppearFromClass = toLegacyClass(appearFromClass)
+    }
+    if (!rawProps.leaveFromClass) {
+      legacyLeaveFromClass = toLegacyClass(leaveFromClass)
     }
-  }
-
-  if (!css) {
-    return baseProps
   }
 
   const durations = normalizeDuration(duration)
@@ -132,6 +157,12 @@ export function resolveTransitionProps(
       hook && hook(el, resolve)
       nextFrame(() => {
         removeTransitionClass(el, isAppear ? appearFromClass : enterFromClass)
+        if (__COMPAT__ && legacyClassEnabled) {
+          removeTransitionClass(
+            el,
+            isAppear ? legacyAppearFromClass : legacyEnterFromClass
+          )
+        }
         addTransitionClass(el, isAppear ? appearToClass : enterToClass)
         if (!(hook && hook.length > 1)) {
           whenTransitionEnds(el, type, enterDuration, resolve)
@@ -144,11 +175,17 @@ export function resolveTransitionProps(
     onBeforeEnter(el) {
       onBeforeEnter && onBeforeEnter(el)
       addTransitionClass(el, enterFromClass)
+      if (__COMPAT__ && legacyClassEnabled) {
+        addTransitionClass(el, legacyEnterFromClass)
+      }
       addTransitionClass(el, enterActiveClass)
     },
     onBeforeAppear(el) {
       onBeforeAppear && onBeforeAppear(el)
       addTransitionClass(el, appearFromClass)
+      if (__COMPAT__ && legacyClassEnabled) {
+        addTransitionClass(el, legacyAppearFromClass)
+      }
       addTransitionClass(el, appearActiveClass)
     },
     onEnter: makeEnterHook(false),
@@ -156,11 +193,17 @@ export function resolveTransitionProps(
     onLeave(el, done) {
       const resolve = () => finishLeave(el, done)
       addTransitionClass(el, leaveFromClass)
+      if (__COMPAT__ && legacyClassEnabled) {
+        addTransitionClass(el, legacyLeaveFromClass)
+      }
       // force reflow so *-leave-from classes immediately take effect (#2593)
       forceReflow()
       addTransitionClass(el, leaveActiveClass)
       nextFrame(() => {
         removeTransitionClass(el, leaveFromClass)
+        if (__COMPAT__ && legacyClassEnabled) {
+          removeTransitionClass(el, legacyLeaveFromClass)
+        }
         addTransitionClass(el, leaveToClass)
         if (!(onLeave && onLeave.length > 1)) {
           whenTransitionEnds(el, type, leaveDuration, resolve)
diff --git a/packages/runtime-dom/src/components/TransitionGroup.ts b/packages/runtime-dom/src/components/TransitionGroup.ts
index 16a6aa39..99f5fa98 100644
--- a/packages/runtime-dom/src/components/TransitionGroup.ts
+++ b/packages/runtime-dom/src/components/TransitionGroup.ts
@@ -20,7 +20,10 @@ import {
   createVNode,
   onUpdated,
   SetupContext,
-  toRaw
+  toRaw,
+  compatUtils,
+  DeprecationTypes,
+  ComponentOptions
 } from '@vue/runtime-core'
 import { extend } from '@vue/shared'
 
@@ -37,7 +40,7 @@ export type TransitionGroupProps = Omit<TransitionProps, 'mode'> & {
   moveClass?: string
 }
 
-const TransitionGroupImpl = {
+const TransitionGroupImpl: ComponentOptions = {
   name: 'TransitionGroup',
 
   props: /*#__PURE__*/ extend({}, TransitionPropsValidators, {
@@ -99,7 +102,19 @@ const TransitionGroupImpl = {
     return () => {
       const rawProps = toRaw(props)
       const cssTransitionProps = resolveTransitionProps(rawProps)
-      const tag = rawProps.tag || Fragment
+      let tag = rawProps.tag || Fragment
+
+      if (
+        __COMPAT__ &&
+        !rawProps.tag &&
+        compatUtils.checkCompatEnabled(
+          DeprecationTypes.TRANSITION_GROUP_ROOT,
+          instance.parent
+        )
+      ) {
+        tag = 'span'
+      }
+
       prevChildren = children
       children = slots.default ? getTransitionRawChildren(slots.default()) : []
 
@@ -131,6 +146,10 @@ const TransitionGroupImpl = {
   }
 }
 
+if (__COMPAT__) {
+  TransitionGroupImpl.__isBuiltIn = true
+}
+
 /**
  * TransitionGroup does not support "mode" so we need to remove it from the
  * props declarations, but direct delete operation is considered a side effect
diff --git a/packages/runtime-dom/src/directives/vOn.ts b/packages/runtime-dom/src/directives/vOn.ts
index ce4a4d0c..45fc58ad 100644
--- a/packages/runtime-dom/src/directives/vOn.ts
+++ b/packages/runtime-dom/src/directives/vOn.ts
@@ -1,4 +1,11 @@
-import { hyphenate } from '@vue/shared'
+import {
+  getCurrentInstance,
+  DeprecationTypes,
+  LegacyConfig,
+  compatUtils,
+  ComponentInternalInstance
+} from '@vue/runtime-core'
+import { hyphenate, isArray } from '@vue/shared'
 
 const systemModifiers = ['ctrl', 'shift', 'alt', 'meta']
 
@@ -51,15 +58,60 @@ const keyNames: Record<string, string | string[]> = {
  * @private
  */
 export const withKeys = (fn: Function, modifiers: string[]) => {
-  return (event: KeyboardEvent) => {
-    if (!('key' in event)) return
-    const eventKey = hyphenate(event.key)
+  let globalKeyCodes: LegacyConfig['keyCodes']
+  let instance: ComponentInternalInstance | null = null
+  if (__COMPAT__) {
+    instance = getCurrentInstance()
     if (
-      // None of the provided key modifiers match the current event key
-      !modifiers.some(k => k === eventKey || keyNames[k] === eventKey)
+      compatUtils.isCompatEnabled(DeprecationTypes.CONFIG_KEY_CODES, instance)
     ) {
+      if (instance) {
+        globalKeyCodes = ((instance.appContext.config as any) as LegacyConfig)
+          .keyCodes
+      }
+    }
+    if (__DEV__ && modifiers.some(m => /^\d+$/.test(m))) {
+      compatUtils.warnDeprecation(
+        DeprecationTypes.V_ON_KEYCODE_MODIFIER,
+        instance
+      )
+    }
+  }
+
+  return (event: KeyboardEvent) => {
+    if (!('key' in event)) {
       return
     }
-    return fn(event)
+
+    const eventKey = hyphenate(event.key)
+    if (modifiers.some(k => k === eventKey || keyNames[k] === eventKey)) {
+      return fn(event)
+    }
+
+    if (__COMPAT__) {
+      const keyCode = String(event.keyCode)
+      if (
+        compatUtils.isCompatEnabled(
+          DeprecationTypes.V_ON_KEYCODE_MODIFIER,
+          instance
+        ) &&
+        modifiers.some(mod => mod == keyCode)
+      ) {
+        return fn(event)
+      }
+      if (globalKeyCodes) {
+        for (const mod of modifiers) {
+          const codes = globalKeyCodes[mod]
+          if (codes) {
+            const matches = isArray(codes)
+              ? codes.some(code => String(code) === keyCode)
+              : String(codes) === keyCode
+            if (matches) {
+              return fn(event)
+            }
+          }
+        }
+      }
+    }
   }
 }
diff --git a/packages/runtime-dom/src/index.ts b/packages/runtime-dom/src/index.ts
index 77347062..2ad8f2b2 100644
--- a/packages/runtime-dom/src/index.ts
+++ b/packages/runtime-dom/src/index.ts
@@ -8,7 +8,9 @@ import {
   HydrationRenderer,
   App,
   RootHydrateFunction,
-  isRuntimeOnly
+  isRuntimeOnly,
+  DeprecationTypes,
+  compatUtils
 } from '@vue/runtime-core'
 import { nodeOps } from './nodeOps'
 import { patchProp, forcePatchProp } from './patchProp'
@@ -56,17 +58,36 @@ export const createApp = ((...args) => {
 
   if (__DEV__) {
     injectNativeTagCheck(app)
-    injectCustomElementCheck(app)
+    injectCompilerOptionsCheck(app)
   }
 
   const { mount } = app
   app.mount = (containerOrSelector: Element | ShadowRoot | string): any => {
     const container = normalizeContainer(containerOrSelector)
     if (!container) return
+
     const component = app._component
     if (!isFunction(component) && !component.render && !component.template) {
+      // __UNSAFE__
+      // Reason: potential execution of JS expressions in in-DOM template.
+      // The user must make sure the in-DOM template is trusted. If it's
+      // rendered by the server, the template should not contain any user data.
       component.template = container.innerHTML
+      // 2.x compat check
+      if (__COMPAT__ && __DEV__) {
+        for (let i = 0; i < container.attributes.length; i++) {
+          const attr = container.attributes[i]
+          if (attr.name !== 'v-cloak' && /^(v-|:|@)/.test(attr.name)) {
+            compatUtils.warnDeprecation(
+              DeprecationTypes.GLOBAL_MOUNT_CONTAINER,
+              null
+            )
+            break
+          }
+        }
+      }
     }
+
     // clear content before mounting
     container.innerHTML = ''
     const proxy = mount(container, false, container instanceof SVGElement)
@@ -85,7 +106,7 @@ export const createSSRApp = ((...args) => {
 
   if (__DEV__) {
     injectNativeTagCheck(app)
-    injectCustomElementCheck(app)
+    injectCompilerOptionsCheck(app)
   }
 
   const { mount } = app
@@ -109,21 +130,40 @@ function injectNativeTagCheck(app: App) {
 }
 
 // dev only
-function injectCustomElementCheck(app: App) {
+function injectCompilerOptionsCheck(app: App) {
   if (isRuntimeOnly()) {
-    const value = app.config.isCustomElement
+    const isCustomElement = app.config.isCustomElement
     Object.defineProperty(app.config, 'isCustomElement', {
       get() {
-        return value
+        return isCustomElement
       },
       set() {
         warn(
-          `The \`isCustomElement\` config option is only respected when using the runtime compiler.` +
-            `If you are using the runtime-only build, \`isCustomElement\` must be passed to \`@vue/compiler-dom\` in the build setup instead` +
-            `- for example, via the \`compilerOptions\` option in vue-loader: https://vue-loader.vuejs.org/options.html#compileroptions.`
+          `The \`isCustomElement\` config option is deprecated. Use ` +
+            `\`compilerOptions.isCustomElement\` instead.`
         )
       }
     })
+
+    const compilerOptions = app.config.compilerOptions
+    const msg =
+      `The \`compilerOptions\` config option is only respected when using ` +
+      `a build of Vue.js that includes the runtime compiler (aka "full build"). ` +
+      `Since you are using the runtime-only build, \`compilerOptions\` ` +
+      `must be passed to \`@vue/compiler-dom\` in the build setup instead.\n` +
+      `- For vue-loader: pass it via vue-loader's \`compilerOptions\` loader option.\n` +
+      `- For vue-cli: see https://cli.vuejs.org/guide/webpack.html#modifying-options-of-a-loader\n` +
+      `- For vite: pass it via @vitejs/plugin-vue options. See https://github.com/vitejs/vite/tree/main/packages/plugin-vue#example-for-passing-options-to-vuecompiler-dom`
+
+    Object.defineProperty(app.config, 'compilerOptions', {
+      get() {
+        warn(msg)
+        return compilerOptions
+      },
+      set() {
+        warn(msg)
+      }
+    })
   }
 }
 
diff --git a/packages/runtime-dom/src/modules/attrs.ts b/packages/runtime-dom/src/modules/attrs.ts
index 3a33d713..fc3ead52 100644
--- a/packages/runtime-dom/src/modules/attrs.ts
+++ b/packages/runtime-dom/src/modules/attrs.ts
@@ -15,6 +15,10 @@ export function patchAttr(
       el.setAttributeNS(xlinkNS, key, value)
     }
   } else {
+    if (__COMPAT__ && compatCoerceAttr(el, key, value)) {
+      return
+    }
+
     // note we are only checking boolean attributes that don't have a
     // corresponding dom prop of the same name here.
     const isBoolean = isSpecialBooleanAttr(key)
@@ -25,3 +29,51 @@ export function patchAttr(
     }
   }
 }
+
+// 2.x compat
+import { makeMap, NOOP } from '@vue/shared'
+import { compatUtils, DeprecationTypes } from '@vue/runtime-core'
+
+const isEnumeratedAttr = __COMPAT__
+  ? /*#__PURE__*/ makeMap('contenteditable,draggable,spellcheck')
+  : NOOP
+
+export function compatCoerceAttr(
+  el: Element,
+  key: string,
+  value: unknown
+): boolean {
+  if (isEnumeratedAttr(key)) {
+    const v2CocercedValue =
+      value === null
+        ? 'false'
+        : typeof value !== 'boolean' && value !== undefined
+          ? 'true'
+          : null
+    if (
+      v2CocercedValue &&
+      compatUtils.softAssertCompatEnabled(
+        DeprecationTypes.ATTR_ENUMERATED_COERSION,
+        null,
+        key,
+        value,
+        v2CocercedValue
+      )
+    ) {
+      el.setAttribute(key, v2CocercedValue)
+      return true
+    }
+  } else if (
+    value === false &&
+    !isSpecialBooleanAttr(key) &&
+    compatUtils.softAssertCompatEnabled(
+      DeprecationTypes.ATTR_FALSE_VALUE,
+      null,
+      key
+    )
+  ) {
+    el.removeAttribute(key)
+    return true
+  }
+  return false
+}
diff --git a/packages/template-explorer/package.json b/packages/template-explorer/package.json
index f7afa8a5..60117ed7 100644
--- a/packages/template-explorer/package.json
+++ b/packages/template-explorer/package.json
@@ -6,6 +6,7 @@
     "formats": [
       "global"
     ],
+    "compat": true,
     "env": "development",
     "enableNonBrowserBranches": true
   },
diff --git a/packages/template-explorer/src/options.ts b/packages/template-explorer/src/options.ts
index e8669bc7..f757a711 100644
--- a/packages/template-explorer/src/options.ts
+++ b/packages/template-explorer/src/options.ts
@@ -8,12 +8,13 @@ export const compilerOptions: CompilerOptions = reactive({
   mode: 'module',
   filename: 'Foo.vue',
   prefixIdentifiers: false,
-  optimizeImports: false,
   hoistStatic: false,
   cacheHandlers: false,
   scopeId: null,
   inline: false,
   ssrCssVars: `{ color }`,
+  compatConfig: { MODE: 3 },
+  whitespace: 'condense',
   bindingMetadata: {
     TestComponent: BindingTypes.SETUP_CONST,
     setupRef: BindingTypes.SETUP_REF,
@@ -83,6 +84,32 @@ const App = {
               h('label', { for: 'mode-function' }, 'function')
             ]),
 
+            // whitespace handling
+            h('li', { id: 'whitespace' }, [
+              h('span', { class: 'label' }, 'whitespace: '),
+              h('input', {
+                type: 'radio',
+                id: 'whitespace-condense',
+                name: 'whitespace',
+                checked: compilerOptions.whitespace === 'condense',
+                onChange() {
+                  compilerOptions.whitespace = 'condense'
+                }
+              }),
+              h('label', { for: 'whitespace-condense' }, 'condense'),
+              ' ',
+              h('input', {
+                type: 'radio',
+                id: 'whitespace-preserve',
+                name: 'whitespace',
+                checked: compilerOptions.whitespace === 'preserve',
+                onChange() {
+                  compilerOptions.whitespace = 'preserve'
+                }
+              }),
+              h('label', { for: 'whitespace-preserve' }, 'preserve')
+            ]),
+
             // SSR
             h('li', [
               h('input', {
@@ -170,18 +197,20 @@ const App = {
               h('label', { for: 'inline' }, 'inline')
             ]),
 
-            // toggle optimizeImports
+            // compat mode
             h('li', [
               h('input', {
                 type: 'checkbox',
-                id: 'optimize-imports',
-                disabled: !isModule || isSSR,
-                checked: isModule && !isSSR && compilerOptions.optimizeImports,
+                id: 'compat',
+                checked: compilerOptions.compatConfig!.MODE === 2,
                 onChange(e: Event) {
-                  compilerOptions.optimizeImports = (e.target as HTMLInputElement).checked
+                  compilerOptions.compatConfig!.MODE = (e.target as HTMLInputElement)
+                    .checked
+                    ? 2
+                    : 3
                 }
               }),
-              h('label', { for: 'optimize-imports' }, 'optimizeImports')
+              h('label', { for: 'compat' }, 'v2 compat mode')
             ])
           ])
         ])
diff --git a/packages/vue-compat/README.md b/packages/vue-compat/README.md
new file mode 100644
index 00000000..62c5aa57
--- /dev/null
+++ b/packages/vue-compat/README.md
@@ -0,0 +1 @@
+# @vue/compat
\ No newline at end of file
diff --git a/packages/vue-compat/api-extractor.json b/packages/vue-compat/api-extractor.json
new file mode 100644
index 00000000..a8982eb0
--- /dev/null
+++ b/packages/vue-compat/api-extractor.json
@@ -0,0 +1,7 @@
+{
+  "extends": "../../api-extractor.json",
+  "mainEntryPointFilePath": "./dist/packages/<unscopedPackageName>/src/index.d.ts",
+  "dtsRollup": {
+    "publicTrimmedFilePath": "./dist/<unscopedPackageName>.d.ts"
+  }
+}
\ No newline at end of file
diff --git a/packages/vue-compat/index.js b/packages/vue-compat/index.js
new file mode 100644
index 00000000..aadbf104
--- /dev/null
+++ b/packages/vue-compat/index.js
@@ -0,0 +1,7 @@
+'use strict'
+
+if (process.env.NODE_ENV === 'production') {
+  module.exports = require('./dist/compat.cjs.prod.js')
+} else {
+  module.exports = require('./dist/compat.cjs.js')
+}
diff --git a/packages/vue-compat/package.json b/packages/vue-compat/package.json
new file mode 100644
index 00000000..205b3fec
--- /dev/null
+++ b/packages/vue-compat/package.json
@@ -0,0 +1,44 @@
+{
+  "name": "@vue/compat",
+  "version": "3.0.11",
+  "description": "Vue 3 compatibility build for Vue 2",
+  "main": "index.js",
+  "module": "dist/vue.runtime.esm-bundler.js",
+  "types": "dist/vue.d.ts",
+  "unpkg": "dist/vue.global.js",
+  "jsdelivr": "dist/vue.global.js",
+  "files": [
+    "index.js",
+    "dist"
+  ],
+  "buildOptions": {
+    "name": "Vue",
+    "filename": "vue",
+    "compat": true,
+    "formats": [
+      "esm-bundler",
+      "esm-bundler-runtime",
+      "cjs",
+      "global",
+      "global-runtime",
+      "esm-browser",
+      "esm-browser-runtime"
+    ]
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/vuejs/vue.git"
+  },
+  "keywords": [
+    "vue"
+  ],
+  "author": "Evan You",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/vuejs/vue/issues"
+  },
+  "homepage": "https://github.com/vuejs/vue/tree/dev/packages/vue-compat#readme",
+  "peerDependencies": {
+    "vue": "3.0.11"
+  }
+}
diff --git a/packages/vue-compat/src/createCompatVue.ts b/packages/vue-compat/src/createCompatVue.ts
new file mode 100644
index 00000000..c9c2b867
--- /dev/null
+++ b/packages/vue-compat/src/createCompatVue.ts
@@ -0,0 +1,46 @@
+// This entry exports the runtime only, and is built as
+// `dist/vue.esm-bundler.js` which is used by default for bundlers.
+import { initDev } from './dev'
+import {
+  compatUtils,
+  createApp,
+  Transition,
+  TransitionGroup,
+  KeepAlive,
+  DeprecationTypes,
+  vShow,
+  vModelDynamic
+} from '@vue/runtime-dom'
+import { extend } from '@vue/shared'
+
+if (__DEV__) {
+  initDev()
+}
+
+import * as runtimeDom from '@vue/runtime-dom'
+
+function wrappedCreateApp(...args: any[]) {
+  // @ts-ignore
+  const app = createApp(...args)
+  if (compatUtils.isCompatEnabled(DeprecationTypes.RENDER_FUNCTION, null)) {
+    // register built-in components so that they can be resolved via strings
+    // in the legacy h() call. The __compat__ prefix is to ensure that v3 h()
+    // doesn't get affected.
+    app.component('__compat__transition', Transition)
+    app.component('__compat__transition-group', TransitionGroup)
+    app.component('__compat__keep-alive', KeepAlive)
+    // built-in directives. No need for prefix since there's no render fn API
+    // for resolving directives via string in v3.
+    app._context.directives.show = vShow
+    app._context.directives.model = vModelDynamic
+  }
+  return app
+}
+
+export function createCompatVue() {
+  const Vue = compatUtils.createCompatVue(wrappedCreateApp)
+  extend(Vue, runtimeDom)
+  // @ts-ignore
+  Vue.createApp = wrappedCreateApp
+  return Vue
+}
diff --git a/packages/vue-compat/src/dev.ts b/packages/vue-compat/src/dev.ts
new file mode 100644
index 00000000..99ba49a2
--- /dev/null
+++ b/packages/vue-compat/src/dev.ts
@@ -0,0 +1,14 @@
+import { initCustomFormatter } from '@vue/runtime-dom'
+
+export function initDev() {
+  if (__BROWSER__) {
+    if (!__ESM_BUNDLER__) {
+      console.info(
+        `You are running a development build of Vue.\n` +
+          `Make sure to use the production build (*.prod.js) when deploying for production.`
+      )
+    }
+
+    initCustomFormatter()
+  }
+}
diff --git a/packages/vue-compat/src/esm-index.ts b/packages/vue-compat/src/esm-index.ts
new file mode 100644
index 00000000..c3caa4a9
--- /dev/null
+++ b/packages/vue-compat/src/esm-index.ts
@@ -0,0 +1,3 @@
+import Vue from './index'
+export default Vue
+export * from '@vue/runtime-dom'
diff --git a/packages/vue-compat/src/esm-runtime.ts b/packages/vue-compat/src/esm-runtime.ts
new file mode 100644
index 00000000..aae95dbe
--- /dev/null
+++ b/packages/vue-compat/src/esm-runtime.ts
@@ -0,0 +1,3 @@
+import Vue from './runtime'
+export default Vue
+export * from '@vue/runtime-dom'
diff --git a/packages/vue-compat/src/index.ts b/packages/vue-compat/src/index.ts
new file mode 100644
index 00000000..bcfb3b62
--- /dev/null
+++ b/packages/vue-compat/src/index.ts
@@ -0,0 +1,97 @@
+// This entry is the "full-build" that includes both the runtime
+// and the compiler, and supports on-the-fly compilation of the template option.
+import { createCompatVue } from './createCompatVue'
+import { compile, CompilerError, CompilerOptions } from '@vue/compiler-dom'
+import { registerRuntimeCompiler, RenderFunction, warn } from '@vue/runtime-dom'
+import { isString, NOOP, generateCodeFrame, extend } from '@vue/shared'
+import { InternalRenderFunction } from 'packages/runtime-core/src/component'
+import * as runtimeDom from '@vue/runtime-dom'
+import {
+  DeprecationTypes,
+  warnDeprecation
+} from '../../runtime-core/src/compat/compatConfig'
+
+const compileCache: Record<string, RenderFunction> = Object.create(null)
+
+function compileToFunction(
+  template: string | HTMLElement,
+  options?: CompilerOptions
+): RenderFunction {
+  if (!isString(template)) {
+    if (template.nodeType) {
+      template = template.innerHTML
+    } else {
+      __DEV__ && warn(`invalid template option: `, template)
+      return NOOP
+    }
+  }
+
+  const key = template
+  const cached = compileCache[key]
+  if (cached) {
+    return cached
+  }
+
+  if (template[0] === '#') {
+    const el = document.querySelector(template)
+    if (__DEV__ && !el) {
+      warn(`Template element not found or is empty: ${template}`)
+    }
+    // __UNSAFE__
+    // Reason: potential execution of JS expressions in in-DOM template.
+    // The user must make sure the in-DOM template is trusted. If it's rendered
+    // by the server, the template should not contain any user data.
+    template = el ? el.innerHTML : ``
+  }
+
+  if (__DEV__ && !__TEST__ && (!options || !options.whitespace)) {
+    warnDeprecation(DeprecationTypes.CONFIG_WHITESPACE, null)
+  }
+
+  const { code } = compile(
+    template,
+    extend(
+      {
+        hoistStatic: true,
+        whitespace: 'preserve',
+        onError: __DEV__ ? onError : undefined,
+        onWarn: __DEV__ ? e => onError(e, true) : NOOP
+      } as CompilerOptions,
+      options
+    )
+  )
+
+  function onError(err: CompilerError, asWarning = false) {
+    const message = asWarning
+      ? err.message
+      : `Template compilation error: ${err.message}`
+    const codeFrame =
+      err.loc &&
+      generateCodeFrame(
+        template as string,
+        err.loc.start.offset,
+        err.loc.end.offset
+      )
+    warn(codeFrame ? `${message}\n${codeFrame}` : message)
+  }
+
+  // The wildcard import results in a huge object with every export
+  // with keys that cannot be mangled, and can be quite heavy size-wise.
+  // In the global build we know `Vue` is available globally so we can avoid
+  // the wildcard object.
+  const render = (__GLOBAL__
+    ? new Function(code)()
+    : new Function('Vue', code)(runtimeDom)) as RenderFunction
+
+  // mark the function as runtime compiled
+  ;(render as InternalRenderFunction)._rc = true
+
+  return (compileCache[key] = render)
+}
+
+registerRuntimeCompiler(compileToFunction)
+
+const Vue = createCompatVue()
+Vue.compile = compileToFunction
+
+export default Vue
diff --git a/packages/vue-compat/src/runtime.ts b/packages/vue-compat/src/runtime.ts
new file mode 100644
index 00000000..76370b84
--- /dev/null
+++ b/packages/vue-compat/src/runtime.ts
@@ -0,0 +1,23 @@
+// This entry exports the runtime only, and is built as
+// `dist/vue.esm-bundler.js` which is used by default for bundlers.
+import { createCompatVue } from './createCompatVue'
+import { warn } from '@vue/runtime-core'
+
+const Vue = createCompatVue()
+
+Vue.compile = (() => {
+  if (__DEV__) {
+    warn(
+      `Runtime compilation is not supported in this build of Vue.` +
+        (__ESM_BUNDLER__
+          ? ` Configure your bundler to alias "vue" to "@vue/compat/dist/vue.esm-bundler.js".`
+          : __ESM_BROWSER__
+            ? ` Use "vue.esm-browser.js" instead.`
+            : __GLOBAL__
+              ? ` Use "vue.global.js" instead.`
+              : ``) /* should not happen */
+    )
+  }
+}) as any
+
+export default Vue
diff --git a/packages/vue/__tests__/runtimeCompilerOptions.spec.ts b/packages/vue/__tests__/runtimeCompilerOptions.spec.ts
new file mode 100644
index 00000000..3222462f
--- /dev/null
+++ b/packages/vue/__tests__/runtimeCompilerOptions.spec.ts
@@ -0,0 +1,104 @@
+import { createApp } from 'vue'
+
+describe('config.compilerOptions', () => {
+  test('isCustomElement', () => {
+    const app = createApp({
+      template: `<foo/>`
+    })
+    app.config.compilerOptions.isCustomElement = (tag: string) => tag === 'foo'
+    const root = document.createElement('div')
+    app.mount(root)
+    expect(root.innerHTML).toBe('<foo></foo>')
+  })
+
+  test('comments', () => {
+    const app = createApp({
+      template: `<div/><!--test--><div/>`
+    })
+    app.config.compilerOptions.comments = true
+    // the comments option is only relevant in production mode
+    __DEV__ = false
+    const root = document.createElement('div')
+    app.mount(root)
+    expect(root.innerHTML).toBe('<div></div><!--test--><div></div>')
+    __DEV__ = true
+  })
+
+  test('whitespace', () => {
+    const app = createApp({
+      template: `<div><span/>\n  <span/></div>`
+    })
+    app.config.compilerOptions.whitespace = 'preserve'
+    const root = document.createElement('div')
+    app.mount(root)
+    expect(root.firstChild!.childNodes.length).toBe(3)
+    expect(root.firstChild!.childNodes[1].nodeType).toBe(Node.TEXT_NODE)
+  })
+
+  test('delimiters', () => {
+    const app = createApp({
+      data: () => ({ foo: 'hi' }),
+      template: `[[ foo ]]`
+    })
+    app.config.compilerOptions.delimiters = [`[[`, `]]`]
+    const root = document.createElement('div')
+    app.mount(root)
+    expect(root.textContent).toBe('hi')
+  })
+})
+
+describe('per-component compilerOptions', () => {
+  test('isCustomElement', () => {
+    const app = createApp({
+      template: `<foo/>`,
+      compilerOptions: {
+        isCustomElement: (tag: string) => tag === 'foo'
+      }
+    })
+    const root = document.createElement('div')
+    app.mount(root)
+    expect(root.innerHTML).toBe('<foo></foo>')
+  })
+
+  test('comments', () => {
+    const app = createApp({
+      template: `<div/><!--test--><div/>`,
+      compilerOptions: {
+        comments: true
+      }
+    })
+    app.config.compilerOptions.comments = false
+    // the comments option is only relevant in production mode
+    __DEV__ = false
+    const root = document.createElement('div')
+    app.mount(root)
+    expect(root.innerHTML).toBe('<div></div><!--test--><div></div>')
+    __DEV__ = true
+  })
+
+  test('whitespace', () => {
+    const app = createApp({
+      template: `<div><span/>\n  <span/></div>`,
+      compilerOptions: {
+        whitespace: 'preserve'
+      }
+    })
+    const root = document.createElement('div')
+    app.mount(root)
+    expect(root.firstChild!.childNodes.length).toBe(3)
+    expect(root.firstChild!.childNodes[1].nodeType).toBe(Node.TEXT_NODE)
+  })
+
+  test('delimiters', () => {
+    const app = createApp({
+      data: () => ({ foo: 'hi' }),
+      template: `[[ foo ]]`,
+      compilerOptions: {
+        delimiters: [`[[`, `]]`]
+      }
+    })
+    const root = document.createElement('div')
+    app.mount(root)
+    expect(root.textContent).toBe('hi')
+  })
+})
diff --git a/packages/vue/package.json b/packages/vue/package.json
index 991561d5..7898508a 100644
--- a/packages/vue/package.json
+++ b/packages/vue/package.json
@@ -1,7 +1,7 @@
 {
   "name": "vue",
   "version": "3.0.11",
-  "description": "vue",
+  "description": "The progressive JavaScript framework for buiding modern web UI.",
   "main": "index.js",
   "module": "dist/vue.runtime.esm-bundler.js",
   "types": "dist/vue.d.ts",
diff --git a/packages/vue/src/index.ts b/packages/vue/src/index.ts
index 4874bb0d..60ea75fd 100644
--- a/packages/vue/src/index.ts
+++ b/packages/vue/src/index.ts
@@ -49,27 +49,27 @@ function compileToFunction(
     extend(
       {
         hoistStatic: true,
-        onError(err: CompilerError) {
-          if (__DEV__) {
-            const message = `Template compilation error: ${err.message}`
-            const codeFrame =
-              err.loc &&
-              generateCodeFrame(
-                template as string,
-                err.loc.start.offset,
-                err.loc.end.offset
-              )
-            warn(codeFrame ? `${message}\n${codeFrame}` : message)
-          } else {
-            /* istanbul ignore next */
-            throw err
-          }
-        }
-      },
+        onError: __DEV__ ? onError : undefined,
+        onWarn: __DEV__ ? e => onError(e, true) : NOOP
+      } as CompilerOptions,
       options
     )
   )
 
+  function onError(err: CompilerError, asWarning = false) {
+    const message = asWarning
+      ? err.message
+      : `Template compilation error: ${err.message}`
+    const codeFrame =
+      err.loc &&
+      generateCodeFrame(
+        template as string,
+        err.loc.start.offset,
+        err.loc.end.offset
+      )
+    warn(codeFrame ? `${message}\n${codeFrame}` : message)
+  }
+
   // The wildcard import results in a huge object with every export
   // with keys that cannot be mangled, and can be quite heavy size-wise.
   // In the global build we know `Vue` is available globally so we can avoid
diff --git a/rollup.config.js b/rollup.config.js
index e8fe8d88..5c5d3b90 100644
--- a/rollup.config.js
+++ b/rollup.config.js
@@ -1,3 +1,4 @@
+// @ts-check
 import path from 'path'
 import ts from 'rollup-plugin-typescript2'
 import replace from '@rollup/plugin-replace'
@@ -10,10 +11,10 @@ if (!process.env.TARGET) {
 const masterVersion = require('./package.json').version
 const packagesDir = path.resolve(__dirname, 'packages')
 const packageDir = path.resolve(packagesDir, process.env.TARGET)
-const name = path.basename(packageDir)
 const resolve = p => path.resolve(packageDir, p)
 const pkg = require(resolve(`package.json`))
 const packageOptions = pkg.buildOptions || {}
+const name = packageOptions.filename || path.basename(packageDir)
 
 // ensure TS checks only once for each build
 let hasTSChecked = false
@@ -89,6 +90,7 @@ function createConfig(format, output, plugins = []) {
   const isBrowserESMBuild = /esm-browser/.test(format)
   const isNodeBuild = format === 'cjs'
   const isGlobalBuild = /global/.test(format)
+  const isCompatBuild = !!packageOptions.compat
 
   if (isGlobalBuild) {
     output.name = packageOptions.name
@@ -114,21 +116,34 @@ function createConfig(format, output, plugins = []) {
   // during a single build.
   hasTSChecked = true
 
-  const entryFile = /runtime$/.test(format) ? `src/runtime.ts` : `src/index.ts`
+  let entryFile = /runtime$/.test(format) ? `src/runtime.ts` : `src/index.ts`
 
-  const external =
-    isGlobalBuild || isBrowserESMBuild
-      ? packageOptions.enableNonBrowserBranches
-        ? []
-        : // normal browser builds - non-browser only imports are tree-shaken,
-          // they are only listed here to suppress warnings.
-          ['source-map', '@babel/parser', 'estree-walker']
-      : // Node / esm-bundler builds. Externalize everything.
-        [
-          ...Object.keys(pkg.dependencies || {}),
-          ...Object.keys(pkg.peerDependencies || {}),
-          ...['path', 'url', 'stream'] // for @vue/compiler-sfc / server-renderer
-        ]
+  // the compat build needs both default AND named exports. This will cause
+  // Rollup to complain for non-ESM targets, so we use separate entries for
+  // esm vs. non-esm builds.
+  if (isCompatBuild && (isBrowserESMBuild || isBundlerESMBuild)) {
+    entryFile = /runtime$/.test(format)
+      ? `src/esm-runtime.ts`
+      : `src/esm-index.ts`
+  }
+
+  let external = []
+
+  if (isGlobalBuild || isBrowserESMBuild || isCompatBuild) {
+    if (!packageOptions.enableNonBrowserBranches) {
+      // normal browser builds - non-browser only imports are tree-shaken,
+      // they are only listed here to suppress warnings.
+      external = ['source-map', '@babel/parser', 'estree-walker']
+    }
+  } else {
+    // Node / esm-bundler builds.
+    // externalize all deps unless it's the compat build.
+    external = [
+      ...Object.keys(pkg.dependencies || {}),
+      ...Object.keys(pkg.peerDependencies || {}),
+      ...['path', 'url', 'stream'] // for @vue/compiler-sfc / server-renderer
+    ]
+  }
 
   // the browser builds of @vue/compiler-sfc requires postcss to be available
   // as a global (e.g. http://wzrd.in/standalone/postcss)
@@ -139,9 +154,11 @@ function createConfig(format, output, plugins = []) {
   const nodePlugins =
     packageOptions.enableNonBrowserBranches && format !== 'cjs'
       ? [
+          // @ts-ignore
           require('@rollup/plugin-commonjs')({
             sourceMap: false
           }),
+          // @ts-ignore
           require('rollup-plugin-polyfill-node')(),
           require('@rollup/plugin-node-resolve').nodeResolve()
         ]
@@ -165,7 +182,8 @@ function createConfig(format, output, plugins = []) {
         (isGlobalBuild || isBrowserESMBuild || isBundlerESMBuild) &&
           !packageOptions.enableNonBrowserBranches,
         isGlobalBuild,
-        isNodeBuild
+        isNodeBuild,
+        isCompatBuild
       ),
       ...nodePlugins,
       ...plugins
@@ -188,7 +206,8 @@ function createReplacePlugin(
   isBrowserESMBuild,
   isBrowserBuild,
   isGlobalBuild,
-  isNodeBuild
+  isNodeBuild,
+  isCompatBuild
 ) {
   const replacements = {
     __COMMIT__: `"${process.env.COMMIT}"`,
@@ -208,6 +227,9 @@ function createReplacePlugin(
     // is targeting Node (SSR)?
     __NODE_JS__: isNodeBuild,
 
+    // 2.x compat build
+    __COMPAT__: isCompatBuild,
+
     // feature flags
     __FEATURE_SUSPENSE__: true,
     __FEATURE_OPTIONS_API__: isBundlerESMBuild ? `__VUE_OPTIONS_API__` : true,
@@ -231,6 +253,7 @@ function createReplacePlugin(
     }
   })
   return replace({
+    // @ts-ignore
     values: replacements,
     preventAssignment: true
   })
diff --git a/scripts/build.js b/scripts/build.js
index 26e287c4..fde0b17c 100644
--- a/scripts/build.js
+++ b/scripts/build.js
@@ -169,6 +169,7 @@ function checkAllSizes(targets) {
 function checkSize(target) {
   const pkgDir = path.resolve(`packages/${target}`)
   checkFileSize(`${pkgDir}/dist/${target}.global.prod.js`)
+  checkFileSize(`${pkgDir}/dist/${target}.runtime.global.prod.js`)
 }
 
 function checkFileSize(filePath) {
diff --git a/tsconfig.json b/tsconfig.json
index 117bce3e..cb22e063 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -18,6 +18,7 @@
     "types": ["jest", "puppeteer", "node"],
     "rootDir": ".",
     "paths": {
+      "@vue/compat": ["packages/vue-compat/src"],
       "@vue/*": ["packages/*/src"],
       "vue": ["packages/vue/src"]
     }