support hydration properly

packages/react-dom-bindings/src/client/ReactDOMComponent.js

@@ -312,7 +312,8 @@ function setInitialDOMProperties(
         // textContent on a <textarea> will cause the placeholder to not
         // show within the <textarea> until it has been focused and blurred again.
         // https://github.com/facebook/react/issues/6731#issuecomment-254874553
-        const canSetTextContent = tag !== 'textarea' || nextProp !== '';
+        const canSetTextContent =
+          tag !== 'body' && (tag !== 'textarea' || nextProp !== '');
         if (canSetTextContent) {
           setTextContent(domElement, nextProp);
         }
@@ -483,18 +484,6 @@ export function createTextNode(
   );
 }
 
-export function resetProperties(
-  domElement: Element,
-  tag: string,
-  rawProps: Object,
-): void {
-  const attributes = domElement.attributes;
-  while (attributes.length) {
-    domElement.removeAttribute(attributes[0].name);
-  }
-  setInitialProperties(domElement, tag, rawProps);
-}
-
 export function setInitialProperties(
   domElement: Element,
   tag: string,

packages/react-dom-bindings/src/client/ReactDOMComponentTree.js

@@ -30,10 +30,7 @@ import {
   SuspenseComponent,
 } from 'react-reconciler/src/ReactWorkTags';
 
-import {
-  getParentSuspenseInstance,
-  supportsSingletons,
-} from './ReactDOMHostConfig';
+import {getParentSuspenseInstance} from './ReactDOMHostConfig';
 
 import {
   enableScopeAPI,
@@ -182,9 +179,7 @@ export function getInstanceFromNode(node: Node): Fiber | null {
       tag === SuspenseComponent ||
       tag === HostRoot ||
       (enableFloat ? tag === HostResource : false) ||
-      (enableHostSingletons && supportsSingletons
-        ? tag === HostSingleton
-        : false)
+      (enableHostSingletons ? tag === HostSingleton : false)
     ) {
       return inst;
     } else {
@@ -204,7 +199,7 @@ export function getNodeFromInstance(inst: Fiber): Instance | TextInstance {
     tag === HostComponent ||
     tag === HostText ||
     (enableFloat ? tag === HostResource : false) ||
-    (enableHostSingletons && supportsSingletons ? tag === HostSingleton : false)
+    (enableHostSingletons ? tag === HostSingleton : false)
   ) {
     // In Fiber this, is just the state node right now. We assume it will be
     // a host component or host text.

packages/react-dom-bindings/src/client/ReactDOMHostConfig.js

@@ -32,7 +32,6 @@ import {
   setInitialProperties,
   diffProperties,
   updateProperties,
-  resetProperties,
   diffHydratedProperties,
   diffHydratedText,
   trapClickOnNonInteractiveElement,
@@ -124,7 +123,6 @@ export type Container =
   | interface extends Document {_reactRootContainer?: FiberRoot}
   | interface extends DocumentFragment {_reactRootContainer?: FiberRoot};
 export type Instance = Element;
-export type InstanceSibling = Node;
 export type TextInstance = Text;
 export interface SuspenseInstance extends Comment {
   _reactRetry?: () => void;
@@ -552,15 +550,15 @@ export function appendChildToContainer(
 export function insertBefore(
   parentInstance: Instance,
   child: Instance | TextInstance,
-  beforeChild: InstanceSibling,
+  beforeChild: Instance,
 ): void {
   parentInstance.insertBefore(child, beforeChild);
 }
 
 export function insertInContainerBefore(
   container: Container,
   child: Instance | TextInstance,
-  beforeChild: InstanceSibling,
+  beforeChild: Instance,
 ): void {
   if (container.nodeType === COMMENT_NODE) {
     (container.parentNode: any).insertBefore(child, beforeChild);
@@ -717,15 +715,32 @@ export function clearContainer(container: Container): void {
         case 'html':
         case 'head':
         case 'body': {
-          clearSingletonInstance(element);
-          break;
+          let node = element.firstChild;
+          while (node) {
+            const nextNode = node.nextSibling;
+            const nodeName = node.nodeName;
+            if (
+              nodeName === 'HEAD' ||
+              nodeName === 'BODY' ||
+              nodeName === 'STYLE' ||
+              (nodeName === 'LINK' &&
+                ((node: any): HTMLLinkElement).rel.toLowerCase() ===
+                  'stylesheet')
+            ) {
+              // retain these nodes
+            } else {
+              element.removeChild(node);
+            }
+            node = nextNode;
+          }
+          return;
         }
         default: {
           element.textContent = '';
         }
       }
     }
-    // Implicitly if the container is of type Document we rely on the Persistent HostComponent
+    // Implicitly if the container is of type Document we rely on the HostSingleton
     // semantics to clear these nodes appropriately when being placed so no ahead of time
     // clearing is necessary
   } else {
@@ -1246,7 +1261,7 @@ export function errorHydratingContainer(parentContainer: Container): void {
   }
 }
 
-export function acquireSingletonInstance(
+export function resolveSingletonInstance(
   type: string,
   props: Props,
   rootContainerInstance: Container,
@@ -1262,50 +1277,51 @@ export function acquireSingletonInstance(
   const ownerDocument = getOwnerDocumentFromRootContainer(
     rootContainerInstance,
   );
-  // For the three persistent Host Components that exist in DOM it is necessary for there to
-  // always be a documentElement. With normal html parsing this will always be the case but
-  // with pathological manipulation the document can end up in a state where no documentElement
-  // exists. We create it here if missing so we can treat it as an invariant.
-  // It is important to note that this dom mutation and others in this function happen
-  // in render rather than commit. This is tolerable because they only happen in degenerate cases
-  let htmlElement = ownerDocument.documentElement;
-  if (!htmlElement) {
-    htmlElement = ownerDocument.appendChild(
-      ownerDocument.createElement('html'),
-    );
-  }
   switch (type) {
     case 'html': {
-      return htmlElement;
+      const documentElement = ownerDocument.documentElement;
+      if (!documentElement) {
+        throw new Error(
+          'React expected an <html> element (document.documentElement) to exist in the Document but one was' +
+            ' not found. React never removes the documentElement for any Document it renders into so' +
+            ' the cause is likely in some other script running on this page.',
+        );
+      }
+      return documentElement;
     }
     case 'head': {
-      let head = ownerDocument.head;
+      const head = ownerDocument.head;
       if (!head) {
-        head = htmlElement.insertBefore(
-          ownerDocument.createElement('head'),
-          ownerDocument.firstChild,
+        throw new Error(
+          'React expected a <head> element (document.head) to exist in the Document but one was' +
+            ' not found. React never removes the head for any Document it renders into so' +
+            ' the cause is likely in some other script running on this page.',
         );
       }
       // We ensure this exists just above
       return head;
     }
     case 'body': {
-      let body = ownerDocument.body;
+      const body = ownerDocument.body;
       if (!body) {
-        body = htmlElement.appendChild(ownerDocument.createElement('body'));
+        throw new Error(
+          'React expected a <body> element (document.body) to exist in the Document but one was' +
+            ' not found. React never removes the body for any Document it renders into so' +
+            ' the cause is likely in some other script running on this page.',
+        );
       }
       // We ensure this exists just above
       return body;
     }
     default: {
       throw new Error(
-        'acquireSingletonInstance was called with an element type that is not supported. This is a bug in React.',
+        'resolveSingletonInstance was called with an element type that is not supported. This is a bug in React.',
       );
     }
   }
 }
 
-export function resetSingletonInstance(
+export function acquireSingletonInstance(
   type: string,
   props: Props,
   instance: Instance,
@@ -1320,51 +1336,17 @@ export function resetSingletonInstance(
       }
       default: {
         console.error(
-          'resetSingletonInstance was called with an element type that is not supported. This is a bug in React.',
+          'acquireSingletonInstance was called with an element type that is not supported. This is a bug in React.',
         );
       }
     }
   }
 
-  clearSingletonInstance(instance);
-  resetProperties(instance, type, props);
+  setInitialProperties(instance, type, props);
   precacheFiberNode(internalInstanceHandle, instance);
   updateFiberProps(instance, props);
 }
 
-export function clearSingletonInstance(instance: Instance) {
-  const tagName = instance.tagName.toLowerCase();
-  switch (tagName) {
-    case 'html': {
-      return;
-    }
-    case 'head':
-    case 'body': {
-      let node = instance.firstChild;
-      while (node) {
-        const nextNode = node.nextSibling;
-        const nodeName = node.nodeName;
-        if (
-          nodeName === 'STYLE' ||
-          (nodeName === 'LINK' &&
-            ((node: any): HTMLLinkElement).rel.toLowerCase() === 'stylesheet')
-        ) {
-          // retain these nodes
-        } else {
-          instance.removeChild(node);
-        }
-        node = nextNode;
-      }
-      return;
-    }
-    default: {
-      throw new Error(
-        'clearSingletonInstance was called with an element type that is not supported. this is a bug in React.',
-      );
-    }
-  }
-}
-
 // -------------------
 //     Test Selectors
 // -------------------

packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js

@@ -4375,7 +4375,7 @@ describe('ReactDOMFizzServer', () => {
     );
   });
 
-  // @gate enableHostSingletons
+  // @gate enableFloat
   it('holds back body and html closing tags (the postamble) until all pending tasks are completed', async () => {
     const chunks = [];
     writable.on('data', chunk => {

packages/react-dom/src/__tests__/ReactDOMRoot-test.js

@@ -358,7 +358,7 @@ describe('ReactDOMRoot', () => {
     );
   });
 
-  // @gate !__DEV__ || !enableHostSingletons
+  // @gate !enableFloat && !enableHostSingletons
   it('warns if updating a root that has had its contents removed', async () => {
     const root = ReactDOMClient.createRoot(container);
     root.render(<div>Hi</div>);