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 bc039500b4ab · 2023-08-22T10:29:07.000-07:00
This change exports a DOCTYPE from FizzConfigDOM and FizzConfigDOMLegacy respectively, using an empty chunk in the legacy case. The only runtime perf cost here is that for legacy APIs there is an extra empty chunk to write when rendering a top level &lt;html&gt; tag which is trivial enough
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;
@@ -3027,7 +3027,10 @@ function startChunkForTag(tag: string): PrecomputedChunk {
   return tagStartChunk;
 }
 
-const DOCTYPE: PrecomputedChunk = stringToPrecomputedChunk('<!DOCTYPE html>');
+export const doctypeChunk: PrecomputedChunk =
+  stringToPrecomputedChunk('<!DOCTYPE html>');
+
+import {doctypeChunk as DOCTYPE} from 'react-server/src/ReactFizzConfig';
 
 export function pushStartInstance(
   target: Array<Chunk | PrecomputedChunk>,
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 {
@@ -104,13 +104,13 @@ export function createResponseState(
   };
 }
 
-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.
-    selectedValue: null,
-    noscriptTagInScope: false,
-  };
-}
+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
+export const doctypeChunk: PrecomputedChunk = stringToPrecomputedChunk('');
 
 export type {
   Resources,
@@ -138,6 +138,7 @@ export {
   writeResourcesForBoundary,
   writePlaceholder,
   writeCompletedRoot,
+  createRootFormatContext,
   createResources,
   createBoundaryResources,
   writePreamble,
@@ -148,8 +149,6 @@ 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>',
+    );
+  });
+});
