diff --git a/packages/react-dom-bindings/src/client/ReactDOMComponentTree.js b/packages/react-dom-bindings/src/client/ReactDOMComponentTree.js
index 7fef1105111..8a07dfefb0f 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMComponentTree.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMComponentTree.js
@@ -38,6 +38,8 @@ import {getParentHydrationBoundary} from './ReactFiberConfigDOM';
 
 import {enableScopeAPI} from 'shared/ReactFeatureFlags';
 
+import {enableInternalInstanceMap} from 'shared/ReactFeatureFlags';
+
 const randomKey = Math.random().toString(36).slice(2);
 const internalInstanceKey = '__reactFiber$' + randomKey;
 const internalPropsKey = '__reactProps$' + randomKey;
@@ -49,7 +51,32 @@ const internalRootNodeResourcesKey = '__reactResources$' + randomKey;
 const internalHoistableMarker = '__reactMarker$' + randomKey;
 const internalScrollTimer = '__reactScroll$' + randomKey;
 
+type InstanceUnion =
+  | Instance
+  | TextInstance
+  | SuspenseInstance
+  | ActivityInstance
+  | ReactScopeInstance
+  | Container;
+
+const PossiblyWeakMap = typeof WeakMap === 'function' ? WeakMap : Map;
+const internalInstanceMap:
+  | WeakMap<InstanceUnion, Fiber>
+  | Map<InstanceUnion, Fiber> = new PossiblyWeakMap();
+const internalPropsMap:
+  | WeakMap<InstanceUnion, Props>
+  | Map<InstanceUnion, Props> = new PossiblyWeakMap();
+
 export function detachDeletedInstance(node: Instance): void {
+  if (enableInternalInstanceMap) {
+    internalInstanceMap.delete(node);
+    internalPropsMap.delete(node);
+    delete (node: any)[internalEventHandlersKey];
+    delete (node: any)[internalEventHandlerListenersKey];
+    delete (node: any)[internalEventHandlesSetKey];
+    delete (node: any)[internalRootNodeResourcesKey];
+    return;
+  }
   // TODO: This function is only called on host components. I don't think all of
   // these fields are relevant.
   delete (node: any)[internalInstanceKey];
@@ -68,6 +95,10 @@ export function precacheFiberNode(
     | ActivityInstance
     | ReactScopeInstance,
 ): void {
+  if (enableInternalInstanceMap) {
+    internalInstanceMap.set(node, hostInst);
+    return;
+  }
   (node: any)[internalInstanceKey] = hostInst;
 }
 
@@ -95,7 +126,12 @@ export function isContainerMarkedAsRoot(node: Container): boolean {
 // HostRoot back. To get to the HostRoot, you need to pass a child of it.
 // The same thing applies to Suspense and Activity boundaries.
 export function getClosestInstanceFromNode(targetNode: Node): null | Fiber {
-  let targetInst = (targetNode: any)[internalInstanceKey];
+  let targetInst: void | Fiber;
+  if (enableInternalInstanceMap) {
+    targetInst = internalInstanceMap.get(((targetNode: any): InstanceUnion));
+  } else {
+    targetInst = (targetNode: any)[internalInstanceKey];
+  }
   if (targetInst) {
     // Don't return HostRoot, SuspenseComponent or ActivityComponent here.
     return targetInst;
@@ -112,9 +148,15 @@ export function getClosestInstanceFromNode(targetNode: Node): null | Fiber {
     // itself because the fibers are conceptually between the container
     // node and the first child. It isn't surrounding the container node.
     // If it's not a container, we check if it's an instance.
-    targetInst =
-      (parentNode: any)[internalContainerInstanceKey] ||
-      (parentNode: any)[internalInstanceKey];
+    if (enableInternalInstanceMap) {
+      targetInst =
+        (parentNode: any)[internalContainerInstanceKey] ||
+        internalInstanceMap.get(((parentNode: any): InstanceUnion));
+    } else {
+      targetInst =
+        (parentNode: any)[internalContainerInstanceKey] ||
+        (parentNode: any)[internalInstanceKey];
+    }
     if (targetInst) {
       // Since this wasn't the direct target of the event, we might have
       // stepped past dehydrated DOM nodes to get here. However they could
@@ -147,8 +189,10 @@ export function getClosestInstanceFromNode(targetNode: Node): null | Fiber {
           // have had an internalInstanceKey on it.
           // Let's get the fiber associated with the SuspenseComponent
           // as the deepest instance.
-          // $FlowFixMe[prop-missing]
-          const targetFiber = hydrationInstance[internalInstanceKey];
+          const targetFiber = enableInternalInstanceMap
+            ? internalInstanceMap.get(hydrationInstance)
+            : // $FlowFixMe[prop-missing]
+              hydrationInstance[internalInstanceKey];
           if (targetFiber) {
             return targetFiber;
           }
@@ -175,9 +219,16 @@ export function getClosestInstanceFromNode(targetNode: Node): null | Fiber {
  * instance, or null if the node was not rendered by this React.
  */
 export function getInstanceFromNode(node: Node): Fiber | null {
-  const inst =
-    (node: any)[internalInstanceKey] ||
-    (node: any)[internalContainerInstanceKey];
+  let inst: void | null | Fiber;
+  if (enableInternalInstanceMap) {
+    inst =
+      internalInstanceMap.get(((node: any): InstanceUnion)) ||
+      (node: any)[internalContainerInstanceKey];
+  } else {
+    inst =
+      (node: any)[internalInstanceKey] ||
+      (node: any)[internalContainerInstanceKey];
+  }
   if (inst) {
     const tag = inst.tag;
     if (
@@ -226,16 +277,24 @@ export function getFiberCurrentPropsFromNode(
     | TextInstance
     | SuspenseInstance
     | ActivityInstance,
-): Props {
+): Props | null {
+  if (enableInternalInstanceMap) {
+    return internalPropsMap.get(node) || null;
+  }
   return (node: any)[internalPropsKey] || null;
 }
 
 export function updateFiberProps(node: Instance, props: Props): void {
+  if (enableInternalInstanceMap) {
+    internalPropsMap.set(node, props);
+    return;
+  }
   (node: any)[internalPropsKey] = props;
 }
 
 export function getEventListenerSet(node: EventTarget): Set<string> {
-  let elementListenerSet = (node: any)[internalEventHandlersKey];
+  let elementListenerSet: Set<string> | void;
+  elementListenerSet = (node: any)[internalEventHandlersKey];
   if (elementListenerSet === undefined) {
     elementListenerSet = (node: any)[internalEventHandlersKey] = new Set();
   }
@@ -246,6 +305,9 @@ export function getFiberFromScopeInstance(
   scope: ReactScopeInstance,
 ): null | Fiber {
   if (enableScopeAPI) {
+    if (enableInternalInstanceMap) {
+      return internalInstanceMap.get(((scope: any): InstanceUnion)) || null;
+    }
     return (scope: any)[internalInstanceKey] || null;
   }
   return null;
@@ -318,6 +380,12 @@ export function clearScrollEndTimer(node: EventTarget): void {
 }
 
 export function isOwnedInstance(node: Node): boolean {
+  if (enableInternalInstanceMap) {
+    return !!(
+      (node: any)[internalHoistableMarker] ||
+      internalInstanceMap.has((node: any))
+    );
+  }
   return !!(
     (node: any)[internalHoistableMarker] || (node: any)[internalInstanceKey]
   );
diff --git a/packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js b/packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js
index bbecbb66356..e4c45ccc4c3 100644
--- a/packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js
+++ b/packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js
@@ -1435,8 +1435,13 @@ export function applyViewTransitionName(
   className: ?string,
 ): void {
   instance = ((instance: any): HTMLElement);
+  // If the name isn't valid CSS identifier, base64 encode the name instead.
+  // This doesn't let you select it in custom CSS selectors but it does work in current
+  // browsers.
+  const escapedName =
+    CSS.escape(name) !== name ? 'r-' + btoa(name).replace(/=/g, '') : name;
   // $FlowFixMe[prop-missing]
-  instance.style.viewTransitionName = name;
+  instance.style.viewTransitionName = escapedName;
   if (className != null) {
     // $FlowFixMe[prop-missing]
     instance.style.viewTransitionClass = className;
diff --git a/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js b/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js
index 4e7fb4b73f6..cac2283236f 100644
--- a/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js
+++ b/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js
@@ -8,7 +8,7 @@ export const clientRenderBoundary =
 export const completeBoundary =
   '$RB=[];$RV=function(a){$RT=performance.now();for(var b=0;b<a.length;b+=2){var c=a[b],e=a[b+1];null!==e.parentNode&&e.parentNode.removeChild(e);var f=c.parentNode;if(f){var g=c.previousSibling,h=0;do{if(c&&8===c.nodeType){var d=c.data;if("/$"===d||"/&"===d)if(0===h)break;else h--;else"$"!==d&&"$?"!==d&&"$~"!==d&&"$!"!==d&&"&"!==d||h++}d=c.nextSibling;f.removeChild(c);c=d}while(c);for(;e.firstChild;)f.insertBefore(e.firstChild,c);g.data="$";g._reactRetry&&requestAnimationFrame(g._reactRetry)}}a.length=0};\n$RC=function(a,b){if(b=document.getElementById(b))(a=document.getElementById(a))?(a.previousSibling.data="$~",$RB.push(a,b),2===$RB.length&&("number"!==typeof $RT?requestAnimationFrame($RV.bind(null,$RB)):(a=performance.now(),setTimeout($RV.bind(null,$RB),2300>a&&2E3<a?2300-a:$RT+300-a)))):b.parentNode.removeChild(b)};';
 export const completeBoundaryUpgradeToViewTransitions =
-  '$RV=function(A,g){function k(a,b){var e=a.getAttribute(b);e&&(b=a.style,l.push(a,b.viewTransitionName,b.viewTransitionClass),"auto"!==e&&(b.viewTransitionClass=e),(a=a.getAttribute("vt-name"))||(a="_T_"+K++ +"_"),b.viewTransitionName=a,B=!0)}var B=!1,K=0,l=[];try{var f=document.__reactViewTransition;if(f){f.finished.finally($RV.bind(null,g));return}var m=new Map;for(f=1;f<g.length;f+=2)for(var h=g[f].querySelectorAll("[vt-share]"),d=0;d<h.length;d++){var c=h[d];m.set(c.getAttribute("vt-name"),c)}var u=[];for(h=0;h<g.length;h+=2){var C=g[h],x=C.parentNode;if(x){var v=x.getBoundingClientRect();if(v.left||v.top||v.width||v.height){c=C;for(f=0;c;){if(8===c.nodeType){var r=c.data;if("/$"===r)if(0===f)break;else f--;else"$"!==r&&"$?"!==r&&"$~"!==r&&"$!"!==r||f++}else if(1===c.nodeType){d=c;var D=d.getAttribute("vt-name"),y=m.get(D);k(d,y?"vt-share":"vt-exit");y&&(k(y,"vt-share"),m.set(D,null));var E=d.querySelectorAll("[vt-share]");for(d=0;d<E.length;d++){var F=E[d],G=F.getAttribute("vt-name"),\nH=m.get(G);H&&(k(F,"vt-share"),k(H,"vt-share"),m.set(G,null))}}c=c.nextSibling}for(var I=g[h+1],t=I.firstElementChild;t;)null!==m.get(t.getAttribute("vt-name"))&&k(t,"vt-enter"),t=t.nextElementSibling;c=x;do for(var n=c.firstElementChild;n;){var J=n.getAttribute("vt-update");J&&"none"!==J&&!l.includes(n)&&k(n,"vt-update");n=n.nextElementSibling}while((c=c.parentNode)&&1===c.nodeType&&"none"!==c.getAttribute("vt-update"));u.push.apply(u,I.querySelectorAll(\'img[src]:not([loading="lazy"])\'))}}}if(B){var z=\ndocument.__reactViewTransition=document.startViewTransition({update:function(){A(g);for(var a=[document.documentElement.clientHeight,document.fonts.ready],b={},e=0;e<u.length;b={g:b.g},e++)if(b.g=u[e],!b.g.complete){var p=b.g.getBoundingClientRect();0<p.bottom&&0<p.right&&p.top<window.innerHeight&&p.left<window.innerWidth&&(p=new Promise(function(w){return function(q){w.g.addEventListener("load",q);w.g.addEventListener("error",q)}}(b)),a.push(p))}return Promise.race([Promise.all(a),new Promise(function(w){var q=\nperformance.now();setTimeout(w,2300>q&&2E3<q?2300-q:500)})])},types:[]});z.ready.finally(function(){for(var a=l.length-3;0<=a;a-=3){var b=l[a],e=b.style;e.viewTransitionName=l[a+1];e.viewTransitionClass=l[a+1];""===b.getAttribute("style")&&b.removeAttribute("style")}});z.finished.finally(function(){document.__reactViewTransition===z&&(document.__reactViewTransition=null)});$RB=[];return}}catch(a){}A(g)}.bind(null,$RV);';
+  '$RV=function(A,g){function k(a,b){var e=a.getAttribute(b);e&&(b=a.style,l.push(a,b.viewTransitionName,b.viewTransitionClass),"auto"!==e&&(b.viewTransitionClass=e),(a=a.getAttribute("vt-name"))||(a="_T_"+K++ +"_"),a=CSS.escape(a)!==a?"r-"+btoa(a).replace(/=/g,""):a,b.viewTransitionName=a,B=!0)}var B=!1,K=0,l=[];try{var f=document.__reactViewTransition;if(f){f.finished.finally($RV.bind(null,g));return}var m=new Map;for(f=1;f<g.length;f+=2)for(var h=g[f].querySelectorAll("[vt-share]"),d=0;d<h.length;d++){var c=h[d];m.set(c.getAttribute("vt-name"),c)}var u=[];for(h=0;h<g.length;h+=2){var C=g[h],x=C.parentNode;if(x){var v=x.getBoundingClientRect();if(v.left||v.top||v.width||v.height){c=C;for(f=0;c;){if(8===c.nodeType){var r=c.data;if("/$"===r)if(0===f)break;else f--;else"$"!==r&&"$?"!==r&&"$~"!==r&&"$!"!==r||f++}else if(1===c.nodeType){d=c;var D=d.getAttribute("vt-name"),y=m.get(D);k(d,y?"vt-share":"vt-exit");y&&(k(y,"vt-share"),m.set(D,null));var E=d.querySelectorAll("[vt-share]");\nfor(d=0;d<E.length;d++){var F=E[d],G=F.getAttribute("vt-name"),H=m.get(G);H&&(k(F,"vt-share"),k(H,"vt-share"),m.set(G,null))}}c=c.nextSibling}for(var I=g[h+1],t=I.firstElementChild;t;)null!==m.get(t.getAttribute("vt-name"))&&k(t,"vt-enter"),t=t.nextElementSibling;c=x;do for(var n=c.firstElementChild;n;){var J=n.getAttribute("vt-update");J&&"none"!==J&&!l.includes(n)&&k(n,"vt-update");n=n.nextElementSibling}while((c=c.parentNode)&&1===c.nodeType&&"none"!==c.getAttribute("vt-update"));u.push.apply(u,\nI.querySelectorAll(\'img[src]:not([loading="lazy"])\'))}}}if(B){var z=document.__reactViewTransition=document.startViewTransition({update:function(){A(g);for(var a=[document.documentElement.clientHeight,document.fonts.ready],b={},e=0;e<u.length;b={g:b.g},e++)if(b.g=u[e],!b.g.complete){var p=b.g.getBoundingClientRect();0<p.bottom&&0<p.right&&p.top<window.innerHeight&&p.left<window.innerWidth&&(p=new Promise(function(w){return function(q){w.g.addEventListener("load",q);w.g.addEventListener("error",q)}}(b)),\na.push(p))}return Promise.race([Promise.all(a),new Promise(function(w){var q=performance.now();setTimeout(w,2300>q&&2E3<q?2300-q:500)})])},types:[]});z.ready.finally(function(){for(var a=l.length-3;0<=a;a-=3){var b=l[a],e=b.style;e.viewTransitionName=l[a+1];e.viewTransitionClass=l[a+1];""===b.getAttribute("style")&&b.removeAttribute("style")}});z.finished.finally(function(){document.__reactViewTransition===z&&(document.__reactViewTransition=null)});$RB=[];return}}catch(a){}A(g)}.bind(null,\n$RV);';
 export const completeBoundaryWithStyles =
   '$RM=new Map;$RR=function(n,w,p){function u(q){this._p=null;q()}for(var r=new Map,t=document,h,b,e=t.querySelectorAll("link[data-precedence],style[data-precedence]"),v=[],k=0;b=e[k++];)"not all"===b.getAttribute("media")?v.push(b):("LINK"===b.tagName&&$RM.set(b.getAttribute("href"),b),r.set(b.dataset.precedence,h=b));e=0;b=[];var l,a;for(k=!0;;){if(k){var f=p[e++];if(!f){k=!1;e=0;continue}var c=!1,m=0;var d=f[m++];if(a=$RM.get(d)){var g=a._p;c=!0}else{a=t.createElement("link");a.href=d;a.rel=\n"stylesheet";for(a.dataset.precedence=l=f[m++];g=f[m++];)a.setAttribute(g,f[m++]);g=a._p=new Promise(function(q,x){a.onload=u.bind(a,q);a.onerror=u.bind(a,x)});$RM.set(d,a)}d=a.getAttribute("media");!g||d&&!matchMedia(d).matches||b.push(g);if(c)continue}else{a=v[e++];if(!a)break;l=a.getAttribute("data-precedence");a.removeAttribute("media")}c=r.get(l)||h;c===h&&(h=a);r.set(l,a);c?c.parentNode.insertBefore(a,c.nextSibling):(c=t.head,c.insertBefore(a,c.firstChild))}if(p=document.getElementById(n))p.previousSibling.data=\n"$~";Promise.all(b).then($RC.bind(null,n,w),$RX.bind(null,n,"CSS failed to load"))};';
 export const completeSegment =
diff --git a/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js b/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js
index 1f27c7e0cdb..efe3ad445bc 100644
--- a/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js
+++ b/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js
@@ -130,7 +130,12 @@ export function revealCompletedBoundariesWithViewTransitions(
       const idPrefix = '';
       name = '_' + idPrefix + 'T_' + autoNameIdx++ + '_';
     }
-    elementStyle['viewTransitionName'] = name;
+    // If the name isn't valid CSS identifier, base64 encode the name instead.
+    // This doesn't let you select it in custom CSS selectors but it does work in current
+    // browsers.
+    const escapedName =
+      CSS.escape(name) !== name ? 'r-' + btoa(name).replace(/=/g, '') : name;
+    elementStyle['viewTransitionName'] = escapedName;
     shouldStartViewTransition = true;
   }
   try {
diff --git a/packages/react-reconciler/src/ReactFiberCommitViewTransitions.js b/packages/react-reconciler/src/ReactFiberCommitViewTransitions.js
index c8e59afb88e..20abaa673c2 100644
--- a/packages/react-reconciler/src/ReactFiberCommitViewTransitions.js
+++ b/packages/react-reconciler/src/ReactFiberCommitViewTransitions.js
@@ -233,7 +233,7 @@ function commitAppearingPairViewTransitions(placement: Fiber): void {
   }
   let child = placement.child;
   while (child !== null) {
-    if (child.tag === OffscreenComponent && child.memoizedState === null) {
+    if (child.tag === OffscreenComponent && child.memoizedState !== null) {
       // This tree was already hidden so we skip it.
     } else {
       commitAppearingPairViewTransitions(child);
@@ -347,7 +347,7 @@ function commitDeletedPairViewTransitions(deletion: Fiber): void {
   }
   let child = deletion.child;
   while (child !== null) {
-    if (child.tag === OffscreenComponent && child.memoizedState === null) {
+    if (child.tag === OffscreenComponent && child.memoizedState !== null) {
       // This tree was already hidden so we skip it.
     } else {
       if (
@@ -550,7 +550,7 @@ function restorePairedViewTransitions(parent: Fiber): void {
   }
   let child = parent.child;
   while (child !== null) {
-    if (child.tag === OffscreenComponent && child.memoizedState === null) {
+    if (child.tag === OffscreenComponent && child.memoizedState !== null) {
       // This tree was already hidden so we skip it.
     } else {
       if (
diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index c31809276bd..a5befa11a0d 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -147,6 +147,8 @@ export const enableFragmentRefs: boolean = true;
 export const enableFragmentRefsScrollIntoView: boolean = true;
 export const enableFragmentRefsInstanceHandles: boolean = false;
 
+export const enableInternalInstanceMap: boolean = false;
+
 // -----------------------------------------------------------------------------
 // Ready for next major.
 //
diff --git a/packages/shared/forks/ReactFeatureFlags.native-fb.js b/packages/shared/forks/ReactFeatureFlags.native-fb.js
index e1b26cdb83b..71c64b9db03 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-fb.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-fb.js
@@ -84,6 +84,7 @@ export const enableComponentPerformanceTrack: boolean =
   __PROFILE__ && dynamicFlags.enableComponentPerformanceTrack;
 export const enablePerformanceIssueReporting: boolean =
   enableComponentPerformanceTrack;
+export const enableInternalInstanceMap: boolean = false;
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);
diff --git a/packages/shared/forks/ReactFeatureFlags.native-oss.js b/packages/shared/forks/ReactFeatureFlags.native-oss.js
index 3880556d247..cf765d7d689 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-oss.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-oss.js
@@ -76,6 +76,8 @@ export const enableFragmentRefs: boolean = true;
 export const enableFragmentRefsScrollIntoView: boolean = false;
 export const enableFragmentRefsInstanceHandles: boolean = false;
 
+export const enableInternalInstanceMap: boolean = false;
+
 // Profiling Only
 export const enableProfilerTimer: boolean = __PROFILE__;
 export const enableProfilerCommitHooks: boolean = __PROFILE__;
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
index 88c46d26e2e..0747f0e8be4 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
@@ -77,6 +77,8 @@ export const enableFragmentRefs: boolean = true;
 export const enableFragmentRefsScrollIntoView: boolean = true;
 export const enableFragmentRefsInstanceHandles: boolean = false;
 
+export const enableInternalInstanceMap: boolean = false;
+
 // TODO: This must be in sync with the main ReactFeatureFlags file because
 // the Test Renderer's value must be the same as the one used by the
 // react package.
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
index d815fd7ddc8..2b40c1b01c6 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
@@ -85,5 +85,7 @@ export const enableFragmentRefsScrollIntoView: boolean = false;
 export const enableFragmentRefsInstanceHandles: boolean = false;
 export const ownerStackLimit = 1e4;
 
+export const enableInternalInstanceMap: boolean = false;
+
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): FeatureFlagsType): ExportsType);
diff --git a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
index 674aa5d3201..1d174609cb9 100644
--- a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
+++ b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
@@ -38,6 +38,8 @@ export const enableFragmentRefs: boolean = __VARIANT__;
 export const enableFragmentRefsScrollIntoView: boolean = __VARIANT__;
 export const enableAsyncDebugInfo: boolean = __VARIANT__;
 
+export const enableInternalInstanceMap: boolean = __VARIANT__;
+
 // TODO: These flags are hard-coded to the default values used in open source.
 // Update the tests so that they pass in either mode, then set these
 // to __VARIANT__.
diff --git a/packages/shared/forks/ReactFeatureFlags.www.js b/packages/shared/forks/ReactFeatureFlags.www.js
index 4ace06ed4c4..029cd0d196e 100644
--- a/packages/shared/forks/ReactFeatureFlags.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.www.js
@@ -35,6 +35,7 @@ export const {
   enableFragmentRefs,
   enableFragmentRefsScrollIntoView,
   enableAsyncDebugInfo,
+  enableInternalInstanceMap,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.
diff --git a/scripts/flow/environment.js b/scripts/flow/environment.js
index f201c2e0e85..180ee3838de 100644
--- a/scripts/flow/environment.js
+++ b/scripts/flow/environment.js
@@ -644,3 +644,7 @@ declare module 'rbush' {
     fromJSON(data: any): RBush<T>;
   }
 }
+
+declare class CSS {
+  static escape(str: string): string;
+}