renderToString is a legacy server API which used a trick to avoid having the DOCTYPE included when rendering full documents by setting the root formatcontext to HTML_MODE rather than ROOT_HTML_MODE. Previously this was of little consequence but with Float the Root mode started to be used for things like determining if we could flush hoistable elements yet. In issue #27177 we see that hoisted elements can appear before the <html> tag when using a legacy API renderToString.

gnoff · gnoff · commit b10b923efe36 · 2023-08-22T09:25:48.000-07:00
This change makes the FormatContext responsible for providing the DOCTYPE chunks rather than the insertionMode. Alongside this the legacy mode now uses the ROOT_HTML_MODE insertionMode so that it follows standard logic for hoisting.

The reason I went with this approach is it allows us to avoid any runtime checks for whether we should or should not include a doctype. The only runtime perf consequence is that for legacy APIs when you render &lt;html&gt; there is an extra empty chunk to write which is of tivial consequence
diff --git a/packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js b/packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js
@@ -371,11 +371,11 @@ export function createResponseState(
 // Constants for the insertion mode we're currently writing in. We don't encode all HTML5 insertion
 // modes. We only include the variants as they matter for the sake of our purposes.
 // We don't actually provide the namespace therefore we use constants instead of the string.
-const ROOT_HTML_MODE = 0; // Used for the root most element tag.
+export const ROOT_HTML_MODE = 0; // Used for the root most element tag.
 // We have a less than HTML_HTML_MODE check elsewhere. If you add more cases here, make sure it
 // still makes sense
 const HTML_HTML_MODE = 1; // Used for the <html> if it is at the top level.
-export const HTML_MODE = 2;
+const HTML_MODE = 2;
 const SVG_MODE = 3;
 const MATHML_MODE = 4;
 const HTML_TABLE_MODE = 5;
@@ -392,6 +392,7 @@ export type FormatContext = {
   insertionMode: InsertionMode, // root/svg/html/mathml/table
   selectedValue: null | string | Array<string>, // the selected value(s) inside a <select>, or null outside <select>
   noscriptTagInScope: boolean,
+  DOCTYPE: PrecomputedChunk,
 };
 
 function createFormatContext(
@@ -403,6 +404,11 @@ function createFormatContext(
     insertionMode,
     selectedValue,
     noscriptTagInScope,
+    // In practice we only need this for the root format context where it conditions the modern/legacy behavior.
+    // However to avoid type variance we include it on all contexts. If we ever read this in legacy mode for any
+    // context other than the root context we would need forward this from parent contexts so the empty doctype
+    // in legacy mode carries forward. In the current setup this isn't necessary
+    DOCTYPE,
   };
 }
 
@@ -2676,11 +2682,12 @@ function pushStartHtml(
   props: Object,
   responseState: ResponseState,
   insertionMode: InsertionMode,
+  doctypeChunk: PrecomputedChunk,
 ): ReactNodeList {
   if (enableFloat) {
     if (insertionMode === ROOT_HTML_MODE && responseState.htmlChunks === null) {
       // This <html> is the Document.documentElement and should be part of the preamble
-      responseState.htmlChunks = [DOCTYPE];
+      responseState.htmlChunks = [doctypeChunk];
       return pushStartGenericElement(responseState.htmlChunks, props, 'html');
     } else {
       // This <html> is deep and is likely just an error. we emit it inline though.
@@ -2692,7 +2699,7 @@ function pushStartHtml(
       // If we're rendering the html tag and we're at the root (i.e. not in foreignObject)
       // then we also emit the DOCTYPE as part of the root content as a convenience for
       // rendering the whole document.
-      target.push(DOCTYPE);
+      target.push(doctypeChunk);
     }
     return pushStartGenericElement(target, props, 'html');
   }
@@ -3196,6 +3203,7 @@ export function pushStartInstance(
         props,
         responseState,
         formatContext.insertionMode,
+        formatContext.DOCTYPE,
       );
     }
     default: {
diff --git a/packages/react-dom-bindings/src/server/ReactFizzConfigDOMLegacy.js b/packages/react-dom-bindings/src/server/ReactFizzConfigDOMLegacy.js
@@ -24,7 +24,7 @@ import {
   writeStartClientRenderedSuspenseBoundary as writeStartClientRenderedSuspenseBoundaryImpl,
   writeEndCompletedSuspenseBoundary as writeEndCompletedSuspenseBoundaryImpl,
   writeEndClientRenderedSuspenseBoundary as writeEndClientRenderedSuspenseBoundaryImpl,
-  HTML_MODE,
+  ROOT_HTML_MODE,
 } from './ReactFizzConfigDOM';
 
 import type {
@@ -104,11 +104,20 @@ export function createResponseState(
   };
 }
 
+import {
+  stringToChunk,
+  stringToPrecomputedChunk,
+} from 'react-server/src/ReactServerStreamConfig';
+
+// this chunk is empty on purpose because we do not want to emit the DOCTYPE in legacy mode
+const DOCTYPE = stringToPrecomputedChunk('');
+
 export function createRootFormatContext(): FormatContext {
   return {
-    insertionMode: HTML_MODE, // We skip the root mode because we don't want to emit the DOCTYPE in legacy mode.
+    insertionMode: ROOT_HTML_MODE,
     selectedValue: null,
     noscriptTagInScope: false,
+    DOCTYPE,
   };
 }
 
@@ -148,8 +157,6 @@ export {
   prepareHostDispatcher,
 } from './ReactFizzConfigDOM';
 
-import {stringToChunk} from 'react-server/src/ReactServerStreamConfig';
-
 import escapeTextForBrowser from './escapeTextForBrowser';
 
 export function pushTextInstance(
diff --git a/packages/react-dom/src/__tests__/ReactDOMLegacyFloat-test.js b/packages/react-dom/src/__tests__/ReactDOMLegacyFloat-test.js
@@ -0,0 +1,41 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ * @jest-environment ./scripts/jest/ReactDOMServerIntegrationEnvironment
+ */
+
+'use strict';
+
+let React;
+let ReactDOMFizzServer;
+
+describe('ReactDOMFloat', () => {
+  beforeEach(() => {
+    jest.resetModules();
+
+    React = require('react');
+    ReactDOMFizzServer = require('react-dom/server');
+  });
+
+  // fixes #27177
+  // @gate enableFloat
+  it('does not hoist above the <html> tag', async () => {
+    const result = ReactDOMFizzServer.renderToString(
+      <html>
+        <head>
+          <script src="foo" />
+          <meta charSet="utf-8" />
+          <title>title</title>
+        </head>
+      </html>,
+    );
+
+    expect(result).toEqual(
+      '<html><head><meta charSet="utf-8"/><title>title</title><script src="foo"></script></head></html>',
+    );
+  });
+});
