diff --git a/packages/react-dom/src/client/ReactDOM.js b/packages/react-dom/src/client/ReactDOM.js
index ee54497f232af..f542aa752b1f5 100644
--- a/packages/react-dom/src/client/ReactDOM.js
+++ b/packages/react-dom/src/client/ReactDOM.js
@@ -56,6 +56,7 @@ import ReactVersion from 'shared/ReactVersion';
 import invariant from 'shared/invariant';
 import {
   exposeConcurrentModeAPIs,
+  disableLegacyReactDOMAPIs,
   disableUnstableCreatePortal,
   disableUnstableRenderSubtreeIntoContainer,
   warnUnstableRenderSubtreeIntoContainer,
@@ -133,12 +134,6 @@ function createPortal(
 const ReactDOM: Object = {
   createPortal,
 
-  // Legacy
-  findDOMNode,
-  hydrate,
-  render,
-  unmountComponentAtNode,
-
   unstable_batchedUpdates: batchedUpdates,
 
   flushSync: flushSync,
@@ -166,6 +161,13 @@ const ReactDOM: Object = {
   version: ReactVersion,
 };
 
+if (!disableLegacyReactDOMAPIs) {
+  ReactDOM.findDOMNode = findDOMNode;
+  ReactDOM.hydrate = hydrate;
+  ReactDOM.render = render;
+  ReactDOM.unmountComponentAtNode = unmountComponentAtNode;
+}
+
 if (exposeConcurrentModeAPIs) {
   ReactDOM.createRoot = createRoot;
   ReactDOM.createBlockingRoot = createBlockingRoot;
diff --git a/packages/react-dom/src/client/ReactDOMFB.js b/packages/react-dom/src/client/ReactDOMFB.js
index 877c7d2ea3f15..5acb45d81e9bb 100644
--- a/packages/react-dom/src/client/ReactDOMFB.js
+++ b/packages/react-dom/src/client/ReactDOMFB.js
@@ -13,19 +13,24 @@ import ReactDOM from './ReactDOM';
 import {isEnabled} from '../events/ReactBrowserEventEmitter';
 import {getClosestInstanceFromNode} from './ReactDOMComponentTree';
 
-Object.assign(
-  (ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: any),
-  {
-    // These are real internal dependencies that are trickier to remove:
-    ReactBrowserEventEmitter: {
-      isEnabled,
+if (__EXPERIMENTAL__) {
+  // This is a modern WWW build.
+  // It should be the same as open source. Don't add new things here.
+} else {
+  // For classic WWW builds, include a few internals that are already in use.
+  Object.assign(
+    (ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: any),
+    {
+      ReactBrowserEventEmitter: {
+        isEnabled,
+      },
+      ReactDOMComponentTree: {
+        getClosestInstanceFromNode,
+      },
+      // Perf experiment
+      addUserTimingListener,
     },
-    ReactDOMComponentTree: {
-      getClosestInstanceFromNode,
-    },
-    // Perf experiment
-    addUserTimingListener,
-  },
-);
+  );
+}
 
 export default ReactDOM;
diff --git a/packages/react-reconciler/src/ReactFiber.js b/packages/react-reconciler/src/ReactFiber.js
index 792cd1def4db5..a51116219927f 100644
--- a/packages/react-reconciler/src/ReactFiber.js
+++ b/packages/react-reconciler/src/ReactFiber.js
@@ -425,7 +425,9 @@ export function createWorkInProgress(
 
     if (__DEV__) {
       // DEV-only fields
-      workInProgress._debugID = current._debugID;
+      if (enableUserTimingAPI) {
+        workInProgress._debugID = current._debugID;
+      }
       workInProgress._debugSource = current._debugSource;
       workInProgress._debugOwner = current._debugOwner;
       workInProgress._debugHookTypes = current._debugHookTypes;
@@ -958,7 +960,9 @@ export function assignFiberPropertiesInDEV(
     target.selfBaseDuration = source.selfBaseDuration;
     target.treeBaseDuration = source.treeBaseDuration;
   }
-  target._debugID = source._debugID;
+  if (enableUserTimingAPI) {
+    target._debugID = source._debugID;
+  }
   target._debugSource = source._debugSource;
   target._debugOwner = source._debugOwner;
   target._debugIsCurrentlyTiming = source._debugIsCurrentlyTiming;
diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index c73997f1dcef9..0dd89f7674a5b 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -114,6 +114,9 @@ export const disableLegacyContext = false;
 // Disables React.createFactory
 export const disableCreateFactory = false;
 
+// Disables hydrate, render, findDOMNode, unmountComponentAtNode
+export const disableLegacyReactDOMAPIs = false;
+
 // Disables children for <textarea> elements
 export const disableTextareaChildren = false;
 
diff --git a/packages/shared/forks/ReactFeatureFlags.native-fb.js b/packages/shared/forks/ReactFeatureFlags.native-fb.js
index c930e0ecc9be9..aad18e8e78b35 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-fb.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-fb.js
@@ -10,7 +10,7 @@
 import invariant from 'shared/invariant';
 
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
-import typeof * as FeatureFlagsShimType from './ReactFeatureFlags.native-fb';
+import typeof * as ExportsType from './ReactFeatureFlags.native-fb';
 
 // Uncomment to re-export dynamic flags from the fbsource version.
 export const {
@@ -46,6 +46,7 @@ export const disableSchedulerTimeoutBasedOnReactExpirationTime = false;
 export const enableTrainModelFix = true;
 export const enableTrustedTypesIntegration = false;
 export const disableCreateFactory = false;
+export const disableLegacyReactDOMAPIs = false;
 export const disableTextareaChildren = false;
 export const disableMapsAsChildren = false;
 export const disableUnstableRenderSubtreeIntoContainer = false;
@@ -63,4 +64,4 @@ export function addUserTimingListener() {
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;
 // eslint-disable-next-line no-unused-expressions
-(null: Check<FeatureFlagsShimType, FeatureFlagsType>);
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/packages/shared/forks/ReactFeatureFlags.native-oss.js b/packages/shared/forks/ReactFeatureFlags.native-oss.js
index ee795fe1e03f0..f25f6d5ff6174 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-oss.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-oss.js
@@ -10,7 +10,7 @@
 import invariant from 'shared/invariant';
 
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
-import typeof * as FeatureFlagsShimType from './ReactFeatureFlags.native-oss';
+import typeof * as ExportsType from './ReactFeatureFlags.native-oss';
 
 export const debugRenderPhaseSideEffectsForStrictMode = false;
 export const enableUserTimingAPI = __DEV__;
@@ -41,6 +41,7 @@ export const enableTrainModelFix = true;
 export const enableTrustedTypesIntegration = false;
 export const enableNativeTargetAsInstance = false;
 export const disableCreateFactory = false;
+export const disableLegacyReactDOMAPIs = false;
 export const disableTextareaChildren = false;
 export const disableMapsAsChildren = false;
 export const disableUnstableRenderSubtreeIntoContainer = false;
@@ -58,4 +59,4 @@ export function addUserTimingListener() {
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;
 // eslint-disable-next-line no-unused-expressions
-(null: Check<FeatureFlagsShimType, FeatureFlagsType>);
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/packages/shared/forks/ReactFeatureFlags.persistent.js b/packages/shared/forks/ReactFeatureFlags.persistent.js
index db307747c1b62..0a20f79fd9e4d 100644
--- a/packages/shared/forks/ReactFeatureFlags.persistent.js
+++ b/packages/shared/forks/ReactFeatureFlags.persistent.js
@@ -10,7 +10,7 @@
 import invariant from 'shared/invariant';
 
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
-import typeof * as PersistentFeatureFlagsType from './ReactFeatureFlags.persistent';
+import typeof * as ExportsType from './ReactFeatureFlags.persistent';
 
 export const debugRenderPhaseSideEffectsForStrictMode = false;
 export const enableUserTimingAPI = __DEV__;
@@ -41,6 +41,7 @@ export const enableTrainModelFix = true;
 export const enableTrustedTypesIntegration = false;
 export const enableNativeTargetAsInstance = false;
 export const disableCreateFactory = false;
+export const disableLegacyReactDOMAPIs = false;
 export const disableTextareaChildren = false;
 export const disableMapsAsChildren = false;
 export const disableUnstableRenderSubtreeIntoContainer = false;
@@ -58,4 +59,4 @@ export function addUserTimingListener() {
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;
 // eslint-disable-next-line no-unused-expressions
-(null: Check<PersistentFeatureFlagsType, FeatureFlagsType>);
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
index 5d8263c720890..15e7314dfe70c 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
@@ -10,7 +10,7 @@
 import invariant from 'shared/invariant';
 
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
-import typeof * as PersistentFeatureFlagsType from './ReactFeatureFlags.persistent';
+import typeof * as ExportsType from './ReactFeatureFlags.test-renderer';
 
 export const debugRenderPhaseSideEffectsForStrictMode = false;
 export const enableUserTimingAPI = __DEV__;
@@ -41,6 +41,7 @@ export const enableTrainModelFix = true;
 export const enableTrustedTypesIntegration = false;
 export const enableNativeTargetAsInstance = false;
 export const disableCreateFactory = false;
+export const disableLegacyReactDOMAPIs = false;
 export const disableTextareaChildren = false;
 export const disableMapsAsChildren = false;
 export const disableUnstableRenderSubtreeIntoContainer = false;
@@ -58,4 +59,4 @@ export function addUserTimingListener() {
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;
 // eslint-disable-next-line no-unused-expressions
-(null: Check<PersistentFeatureFlagsType, FeatureFlagsType>);
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
index ee17330bb4746..924aece014e8e 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
@@ -10,7 +10,7 @@
 import invariant from 'shared/invariant';
 
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
-import typeof * as PersistentFeatureFlagsType from './ReactFeatureFlags.persistent';
+import typeof * as ExportsType from './ReactFeatureFlags.test-renderer.www';
 
 export const debugRenderPhaseSideEffectsForStrictMode = false;
 export const enableUserTimingAPI = __DEV__;
@@ -22,8 +22,10 @@ export const enableSuspenseServerRenderer = false;
 export const enableSelectiveHydration = false;
 export const enableChunksAPI = false;
 export const exposeConcurrentModeAPIs = __EXPERIMENTAL__;
+export const warnAboutShorthandPropertyCollision = true;
 export const enableSchedulerDebugging = false;
 export const disableJavaScriptURLs = false;
+export const disableInputAttributeSyncing = false;
 export const enableDeprecatedFlareAPI = true;
 export const enableFundamentalAPI = false;
 export const enableScopeAPI = true;
@@ -39,6 +41,7 @@ export const enableTrainModelFix = true;
 export const enableTrustedTypesIntegration = false;
 export const enableNativeTargetAsInstance = false;
 export const disableCreateFactory = false;
+export const disableLegacyReactDOMAPIs = false;
 export const disableTextareaChildren = false;
 export const disableMapsAsChildren = false;
 export const disableUnstableRenderSubtreeIntoContainer = false;
@@ -56,4 +59,4 @@ export function addUserTimingListener() {
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;
 // eslint-disable-next-line no-unused-expressions
-(null: Check<PersistentFeatureFlagsType, FeatureFlagsType>);
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/packages/shared/forks/ReactFeatureFlags.testing.js b/packages/shared/forks/ReactFeatureFlags.testing.js
index 13fcc7e198011..64af39fb501ea 100644
--- a/packages/shared/forks/ReactFeatureFlags.testing.js
+++ b/packages/shared/forks/ReactFeatureFlags.testing.js
@@ -10,7 +10,7 @@
 import invariant from 'shared/invariant';
 
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
-import typeof * as PersistentFeatureFlagsType from './ReactFeatureFlags.persistent';
+import typeof * as ExportsType from './ReactFeatureFlags.testing';
 
 export const debugRenderPhaseSideEffectsForStrictMode = false;
 export const enableUserTimingAPI = __DEV__;
@@ -41,6 +41,7 @@ export const enableTrainModelFix = true;
 export const enableTrustedTypesIntegration = false;
 export const enableNativeTargetAsInstance = false;
 export const disableCreateFactory = false;
+export const disableLegacyReactDOMAPIs = false;
 export const disableTextareaChildren = false;
 export const disableMapsAsChildren = false;
 export const disableUnstableRenderSubtreeIntoContainer = false;
@@ -58,4 +59,4 @@ export function addUserTimingListener() {
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;
 // eslint-disable-next-line no-unused-expressions
-(null: Check<PersistentFeatureFlagsType, FeatureFlagsType>);
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/packages/shared/forks/ReactFeatureFlags.testing.www.js b/packages/shared/forks/ReactFeatureFlags.testing.www.js
new file mode 100644
index 0000000000000..1bc7b9f078850
--- /dev/null
+++ b/packages/shared/forks/ReactFeatureFlags.testing.www.js
@@ -0,0 +1,62 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import invariant from 'shared/invariant';
+
+import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
+import typeof * as ExportsType from './ReactFeatureFlags.testing.www';
+
+export const debugRenderPhaseSideEffectsForStrictMode = false;
+export const enableUserTimingAPI = false;
+export const warnAboutDeprecatedLifecycles = true;
+export const replayFailedUnitOfWorkWithInvokeGuardedCallback = false;
+export const enableProfilerTimer = false;
+export const enableSchedulerTracing = false;
+export const enableSuspenseServerRenderer = true;
+export const enableSelectiveHydration = true;
+export const enableChunksAPI = true;
+export const disableJavaScriptURLs = true;
+export const disableInputAttributeSyncing = false;
+export const exposeConcurrentModeAPIs = true;
+export const warnAboutShorthandPropertyCollision = true;
+export const enableSchedulerDebugging = false;
+export const enableDeprecatedFlareAPI = true;
+export const enableFundamentalAPI = false;
+export const enableScopeAPI = true;
+export const enableJSXTransformAPI = true;
+export const warnAboutUnmockedScheduler = true;
+export const flushSuspenseFallbacksInTests = true;
+export const enableSuspenseCallback = true;
+export const warnAboutDefaultPropsOnFunctionComponents = false;
+export const warnAboutStringRefs = false;
+export const disableLegacyContext = __EXPERIMENTAL__;
+export const disableSchedulerTimeoutBasedOnReactExpirationTime = false;
+export const enableTrainModelFix = true;
+export const enableTrustedTypesIntegration = false;
+export const enableNativeTargetAsInstance = false;
+export const disableCreateFactory = __EXPERIMENTAL__;
+export const disableLegacyReactDOMAPIs = __EXPERIMENTAL__;
+export const disableTextareaChildren = __EXPERIMENTAL__;
+export const disableMapsAsChildren = __EXPERIMENTAL__;
+export const disableUnstableRenderSubtreeIntoContainer = __EXPERIMENTAL__;
+export const warnUnstableRenderSubtreeIntoContainer = false;
+export const disableUnstableCreatePortal = __EXPERIMENTAL__;
+export const deferPassiveEffectCleanupDuringUnmount = false;
+export const isTestEnvironment = true;
+
+// Only used in www builds.
+export function addUserTimingListener() {
+  invariant(false, 'Not implemented.');
+}
+
+// Flow magic to verify the exports of this file match the original version.
+// eslint-disable-next-line no-unused-vars
+type Check<_X, Y: _X, X: Y = _X> = null;
+// eslint-disable-next-line no-unused-expressions
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/packages/shared/forks/ReactFeatureFlags.www.js b/packages/shared/forks/ReactFeatureFlags.www.js
index 74f346b017b52..eff0275c99f19 100644
--- a/packages/shared/forks/ReactFeatureFlags.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.www.js
@@ -8,7 +8,7 @@
  */
 
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
-import typeof * as FeatureFlagsShimType from './ReactFeatureFlags.www';
+import typeof * as ExportsType from './ReactFeatureFlags.www';
 
 // Re-export dynamic flags from the www version.
 export const {
@@ -19,13 +19,16 @@ export const {
   warnAboutShorthandPropertyCollision,
 } = require('ReactFeatureFlags');
 
+// On WWW, __EXPERIMENTAL__ is used for a new modern build.
+// It's not used anywhere in production yet.
+
 // In www, we have experimental support for gathering data
 // from User Timing API calls in production. By default, we
 // only emit performance.mark/measure calls in __DEV__. But if
 // somebody calls addUserTimingListener() which is exposed as an
 // experimental FB-only export, we call performance.mark/measure
 // as long as there is more than a single listener.
-export let enableUserTimingAPI = __DEV__;
+export let enableUserTimingAPI = __DEV__ && !__EXPERIMENTAL__;
 
 export const enableProfilerTimer = __PROFILE__;
 export const enableSchedulerTracing = __PROFILE__;
@@ -33,19 +36,19 @@ export const enableSchedulerDebugging = true;
 
 export const replayFailedUnitOfWorkWithInvokeGuardedCallback = false;
 export const warnAboutDeprecatedLifecycles = true;
-export const disableLegacyContext = false;
+export const disableLegacyContext = __EXPERIMENTAL__;
 export const warnAboutStringRefs = false;
 export const warnAboutDefaultPropsOnFunctionComponents = false;
 export const disableSchedulerTimeoutBasedOnReactExpirationTime = false;
 
 export const enableTrainModelFix = true;
 
-export const exposeConcurrentModeAPIs = __EXPERIMENTAL__;
+export const exposeConcurrentModeAPIs = true;
 
 export const enableSuspenseServerRenderer = true;
 export const enableSelectiveHydration = true;
 
-export const enableChunksAPI = __EXPERIMENTAL__;
+export const enableChunksAPI = true;
 
 export const disableJavaScriptURLs = true;
 
@@ -92,17 +95,19 @@ export const flushSuspenseFallbacksInTests = true;
 
 export const enableNativeTargetAsInstance = false;
 
-export const disableCreateFactory = false;
+export const disableCreateFactory = __EXPERIMENTAL__;
+
+export const disableLegacyReactDOMAPIs = __EXPERIMENTAL__;
 
-export const disableTextareaChildren = false;
+export const disableTextareaChildren = __EXPERIMENTAL__;
 
-export const disableMapsAsChildren = false;
+export const disableMapsAsChildren = __EXPERIMENTAL__;
 
-export const disableUnstableRenderSubtreeIntoContainer = false;
+export const disableUnstableRenderSubtreeIntoContainer = __EXPERIMENTAL__;
 
 export const warnUnstableRenderSubtreeIntoContainer = false;
 
-export const disableUnstableCreatePortal = false;
+export const disableUnstableCreatePortal = __EXPERIMENTAL__;
 
 export const isTestEnvironment = false;
 
@@ -110,4 +115,4 @@ export const isTestEnvironment = false;
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;
 // eslint-disable-next-line no-unused-expressions
-(null: Check<FeatureFlagsShimType, FeatureFlagsType>);
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/scripts/rollup/build.js b/scripts/rollup/build.js
index 97fee0eab2853..efdc6bef1b35f 100644
--- a/scripts/rollup/build.js
+++ b/scripts/rollup/build.js
@@ -679,17 +679,17 @@ async function buildEverything() {
       [bundle, NODE_DEV],
       [bundle, NODE_PROD],
       [bundle, NODE_PROFILING],
+      [bundle, FB_WWW_DEV],
+      [bundle, FB_WWW_PROD],
+      [bundle, FB_WWW_PROFILING],
       [bundle, RN_OSS_DEV],
       [bundle, RN_OSS_PROD],
       [bundle, RN_OSS_PROFILING]
     );
 
     if (__EXPERIMENTAL__) {
-      // FB specific builds are experimental-only.
+      // FB-specific RN builds are experimental-only.
       bundles.push(
-        [bundle, FB_WWW_DEV],
-        [bundle, FB_WWW_PROD],
-        [bundle, FB_WWW_PROFILING],
         [bundle, RN_FB_DEV],
         [bundle, RN_FB_PROD],
         [bundle, RN_FB_PROFILING]
diff --git a/scripts/rollup/forks.js b/scripts/rollup/forks.js
index 9f1f3c41bdd62..27b3c97a51511 100644
--- a/scripts/rollup/forks.js
+++ b/scripts/rollup/forks.js
@@ -107,8 +107,13 @@ const forks = Object.freeze({
         }
         return 'shared/forks/ReactFeatureFlags.test-renderer.js';
       case 'react-dom/testing':
-        return 'shared/forks/ReactFeatureFlags.testing.js';
       case 'react/testing':
+        switch (bundleType) {
+          case FB_WWW_DEV:
+          case FB_WWW_PROD:
+          case FB_WWW_PROFILING:
+            return 'shared/forks/ReactFeatureFlags.testing.www.js';
+        }
         return 'shared/forks/ReactFeatureFlags.testing.js';
       default:
         switch (bundleType) {