From 3d4fec978822fad1275f8fe41a2d8664ebe84560 Mon Sep 17 00:00:00 2001
From: Sebastian Markbage <sebastian@calyptus.eu>
Date: Wed, 19 Jun 2024 15:15:42 +0200
Subject: [PATCH 1/8] Add react-html package skeleton

---
 packages/react-html/README.md                 | 32 +++++++++++++++++
 packages/react-html/index.js                  |  5 +++
 packages/react-html/npm/index.js              |  5 +++
 .../react-html/npm/react-html.react-server.js |  7 ++++
 packages/react-html/package.json              | 36 +++++++++++++++++++
 .../react-html/react-html.react-server.js     | 10 ++++++
 packages/react-html/src/ReactHTMLServer.js    | 25 +++++++++++++
 .../src/__tests__/ReactHTMLServer-test.js     | 36 +++++++++++++++++++
 scripts/error-codes/codes.json                |  3 +-
 9 files changed, 158 insertions(+), 1 deletion(-)
 create mode 100644 packages/react-html/README.md
 create mode 100644 packages/react-html/index.js
 create mode 100644 packages/react-html/npm/index.js
 create mode 100644 packages/react-html/npm/react-html.react-server.js
 create mode 100644 packages/react-html/package.json
 create mode 100644 packages/react-html/react-html.react-server.js
 create mode 100644 packages/react-html/src/ReactHTMLServer.js
 create mode 100644 packages/react-html/src/__tests__/ReactHTMLServer-test.js

diff --git a/packages/react-html/README.md b/packages/react-html/README.md
new file mode 100644
index 0000000000000..a4acd1fd84396
--- /dev/null
+++ b/packages/react-html/README.md
@@ -0,0 +1,32 @@
+# `react-html`
+
+This package provides the ability to render standalone HTML from Server Components for use in embedded contexts such as e-mails and RSS/Atom feeds. It cannot use Client Components and aren't hydrated. It is intended to be paired with the generic React package, which is shipped as `react` to npm.
+
+## Installation
+
+```sh
+npm install react react-html
+```
+
+## Usage
+
+```js
+import { renderToMarkup } from 'react-html';
+import EmailTemplate from './my-email-template-component.js'
+
+async function action(email, name) {
+  "use server";
+  // ... in your server, e.g. a Server Action...
+  const htmlString = await renderToMarkup(<EmailTemplate name={name} />);
+  // ... send e-mail using some e-mail provider
+  await sendEmail({ to: email, contentType: 'text/html', body: htmlString });
+}
+```
+
+Note that this is an async function that need to be awaited - unlike the legacy `renderToString` in `react-dom`.
+
+## API
+
+### `react-html`
+
+See https://react.dev/reference/react-html
diff --git a/packages/react-html/index.js b/packages/react-html/index.js
new file mode 100644
index 0000000000000..a1818e8c3bba6
--- /dev/null
+++ b/packages/react-html/index.js
@@ -0,0 +1,5 @@
+'use strict';
+
+throw new Error(
+  'react-html is not supported outside a React Server Components environment.',
+);
diff --git a/packages/react-html/npm/index.js b/packages/react-html/npm/index.js
new file mode 100644
index 0000000000000..e567bb2c0aa21
--- /dev/null
+++ b/packages/react-html/npm/index.js
@@ -0,0 +1,5 @@
+'use strict';
+
+throw new Error(
+  'react-html is not supported outside a React Server Components environment.'
+);
diff --git a/packages/react-html/npm/react-html.react-server.js b/packages/react-html/npm/react-html.react-server.js
new file mode 100644
index 0000000000000..be2af1e0c5838
--- /dev/null
+++ b/packages/react-html/npm/react-html.react-server.js
@@ -0,0 +1,7 @@
+'use strict';
+
+if (process.env.NODE_ENV === 'production') {
+  module.exports = require('./cjs/react-html.react-server.production.js');
+} else {
+  module.exports = require('./cjs/react-html.react-server.development.js');
+}
diff --git a/packages/react-html/package.json b/packages/react-html/package.json
new file mode 100644
index 0000000000000..e175945ecf8fb
--- /dev/null
+++ b/packages/react-html/package.json
@@ -0,0 +1,36 @@
+{
+  "name": "react-html",
+  "version": "19.0.0",
+  "private": true,
+  "description": "React package generating embedded HTML markup such as e-mails using Server Components.",
+  "main": "index.js",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/facebook/react.git",
+    "directory": "packages/react-html"
+  },
+  "keywords": [
+    "react"
+  ],
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/facebook/react/issues"
+  },
+  "homepage": "https://react.dev/",
+  "peerDependencies": {
+    "react": "^19.0.0"
+  },
+  "files": [
+    "LICENSE",
+    "README.md",
+    "index.js",
+    "react-html.react-server.js",
+    "cjs/"
+  ],
+  "exports": {
+    ".": {
+      "react-server": "./react-dom.react-server.js",
+      "default": "./index.js"
+    }
+  }
+}
diff --git a/packages/react-html/react-html.react-server.js b/packages/react-html/react-html.react-server.js
new file mode 100644
index 0000000000000..fb30cd4200836
--- /dev/null
+++ b/packages/react-html/react-html.react-server.js
@@ -0,0 +1,10 @@
+/**
+ * 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.
+ *
+ * @flow
+ */
+
+export * from './src/ReactHTMLServer';
diff --git a/packages/react-html/src/ReactHTMLServer.js b/packages/react-html/src/ReactHTMLServer.js
new file mode 100644
index 0000000000000..09a179a612f30
--- /dev/null
+++ b/packages/react-html/src/ReactHTMLServer.js
@@ -0,0 +1,25 @@
+/**
+ * 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.
+ *
+ * @flow
+ */
+
+import type {ReactNodeList} from 'shared/ReactTypes';
+
+import ReactVersion from 'shared/ReactVersion';
+
+type MarkupOptions = {
+  identifierPrefix?: string,
+};
+
+export function renderToMarkup(
+  children: ReactNodeList,
+  options?: MarkupOptions,
+): Promise<string> {
+  return Promise.resolve('hi');
+}
+
+export {ReactVersion as version};
diff --git a/packages/react-html/src/__tests__/ReactHTMLServer-test.js b/packages/react-html/src/__tests__/ReactHTMLServer-test.js
new file mode 100644
index 0000000000000..b1d2404e09d90
--- /dev/null
+++ b/packages/react-html/src/__tests__/ReactHTMLServer-test.js
@@ -0,0 +1,36 @@
+/**
+ * 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
+ */
+
+'use strict';
+
+let React;
+let ReactHTML;
+
+describe('ReactHTML', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    // We run in the react-server condition.
+    jest.mock('react', () => require('react/react.react-server'));
+    jest.mock('react-html', () =>
+      require('react-html/react-html.react-server'),
+    );
+
+    React = require('react');
+    ReactHTML = require('react-html');
+  });
+
+  it('should be able to render a simple component', async () => {
+    function Component() {
+      return <div>hello world</div>;
+    }
+
+    const html = await ReactHTML.renderToMarkup(<Component />);
+    expect(html).toBe('hi');
+  });
+});
diff --git a/scripts/error-codes/codes.json b/scripts/error-codes/codes.json
index ef4ae75a6d634..63c0e7061e882 100644
--- a/scripts/error-codes/codes.json
+++ b/scripts/error-codes/codes.json
@@ -515,5 +515,6 @@
   "527": "Incompatible React versions: The \"react\" and \"react-dom\" packages must have the exact same version. Instead got:\n  - react:      %s\n  - react-dom:  %s\nLearn more: https://react.dev/warnings/version-mismatch",
   "528": "Expected <link> not to update to be updated to a stylesheet with precedence. Check the `rel`, `href`, and `precedence` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.%s",
   "529": "Expected stylesheet with precedence to not be updated to a different kind of <link>. Check the `rel`, `href`, and `precedence` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.%s",
-  "530": "The render was aborted by the server with a promise."
+  "530": "The render was aborted by the server with a promise.",
+  "531": "react-html is not supported outside a React Server Components environment."
 }

From a7df40bc6f5de3f2e63dd8cda0e1d34a453586fa Mon Sep 17 00:00:00 2001
From: Sebastian Markbage <sebastian@calyptus.eu>
Date: Wed, 19 Jun 2024 20:02:22 +0200
Subject: [PATCH 2/8] Implement renderMarkup as a Fizz renderer

---
 packages/react-html/src/ReactHTMLServer.js    | 80 ++++++++++++++++++-
 .../src/__tests__/ReactHTMLServer-test.js     |  2 +-
 scripts/shared/inlinedHostConfigs.js          |  2 +
 3 files changed, 82 insertions(+), 2 deletions(-)

diff --git a/packages/react-html/src/ReactHTMLServer.js b/packages/react-html/src/ReactHTMLServer.js
index 09a179a612f30..440f7ae337d0e 100644
--- a/packages/react-html/src/ReactHTMLServer.js
+++ b/packages/react-html/src/ReactHTMLServer.js
@@ -9,17 +9,95 @@
 
 import type {ReactNodeList} from 'shared/ReactTypes';
 
+import type {
+  Request,
+  PostponedState,
+  ErrorInfo,
+} from 'react-server/src/ReactFizzServer';
+
 import ReactVersion from 'shared/ReactVersion';
 
+import {
+  createRequest,
+  startWork,
+  startFlowing,
+  abort,
+} from 'react-server/src/ReactFizzServer';
+
+import {
+  createResumableState,
+  createRenderState,
+  createRootFormatContext,
+} from 'react-dom-bindings/src/server/ReactFizzConfigDOMLegacy';
+
 type MarkupOptions = {
   identifierPrefix?: string,
+  signal?: AbortSignal,
 };
 
 export function renderToMarkup(
   children: ReactNodeList,
   options?: MarkupOptions,
 ): Promise<string> {
-  return Promise.resolve('hi');
+  return new Promise((resolve, reject) => {
+    let didFatal = false;
+    let fatalError = null;
+    let buffer = '';
+    const destination = {
+      // $FlowFixMe[missing-local-annot]
+      push(chunk) {
+        if (chunk !== null) {
+          buffer += chunk;
+        } else {
+          // null indicates that we finished
+          resolve(buffer);
+        }
+        return true;
+      },
+      // $FlowFixMe[missing-local-annot]
+      destroy(error) {
+        reject(error);
+      },
+    };
+    function onError(error: mixed) {
+      // Any error rejects the promise, regardless of where it happened.
+      // Unlike other React SSR we don't want to put Suspense boundaries into
+      // client rendering mode because there's no client rendering here.
+      reject(error);
+    }
+    const resumableState = createResumableState(
+      options ? options.identifierPrefix : undefined,
+      undefined,
+    );
+    const request = createRequest(
+      children,
+      resumableState,
+      createRenderState(resumableState, true),
+      createRootFormatContext(),
+      Infinity,
+      onError,
+      undefined,
+      undefined,
+      undefined,
+      undefined,
+      undefined,
+      undefined,
+    );
+    if (options && options.signal) {
+      const signal = options.signal;
+      if (signal.aborted) {
+        abort(request, (signal: any).reason);
+      } else {
+        const listener = () => {
+          abort(request, (signal: any).reason);
+          signal.removeEventListener('abort', listener);
+        };
+        signal.addEventListener('abort', listener);
+      }
+    }
+    startWork(request);
+    startFlowing(request, destination);
+  });
 }
 
 export {ReactVersion as version};
diff --git a/packages/react-html/src/__tests__/ReactHTMLServer-test.js b/packages/react-html/src/__tests__/ReactHTMLServer-test.js
index b1d2404e09d90..592162663308e 100644
--- a/packages/react-html/src/__tests__/ReactHTMLServer-test.js
+++ b/packages/react-html/src/__tests__/ReactHTMLServer-test.js
@@ -31,6 +31,6 @@ describe('ReactHTML', () => {
     }
 
     const html = await ReactHTML.renderToMarkup(<Component />);
-    expect(html).toBe('hi');
+    expect(html).toBe('<div>hello world</div>');
   });
 });
diff --git a/scripts/shared/inlinedHostConfigs.js b/scripts/shared/inlinedHostConfigs.js
index a63da44c36f29..d138e1acc8568 100644
--- a/scripts/shared/inlinedHostConfigs.js
+++ b/scripts/shared/inlinedHostConfigs.js
@@ -395,6 +395,7 @@ module.exports = [
     entryPoints: [
       'react-dom/src/server/ReactDOMLegacyServerBrowser.js', // react-dom/server.browser
       'react-dom/src/server/ReactDOMLegacyServerNode.js', // react-dom/server.node
+      'react-html/react-html.react-server.js',
     ],
     paths: [
       'react-dom',
@@ -404,6 +405,7 @@ module.exports = [
       'react-dom/src/server/ReactDOMLegacyServerImpl.js', // not an entrypoint, but only usable in *Browser and *Node files
       'react-dom/src/server/ReactDOMLegacyServerBrowser.js', // react-dom/server.browser
       'react-dom/src/server/ReactDOMLegacyServerNode.js', // react-dom/server.node
+      'react-html',
       'shared/ReactDOMSharedInternals',
     ],
     isFlowTyped: true,

From 4def38f98d24e165c8e6e0802d815e23d36a4d29 Mon Sep 17 00:00:00 2001
From: Sebastian Markbage <sebastian@calyptus.eu>
Date: Thu, 20 Jun 2024 10:20:15 +0200
Subject: [PATCH 3/8] Add bundle

---
 packages/react-html/package.json     |  4 +++-
 scripts/rollup/bundles.js            | 13 +++++++++++++
 scripts/rollup/forks.js              |  9 ++++++++-
 scripts/shared/inlinedHostConfigs.js |  2 +-
 4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/packages/react-html/package.json b/packages/react-html/package.json
index e175945ecf8fb..c9c4205304b53 100644
--- a/packages/react-html/package.json
+++ b/packages/react-html/package.json
@@ -31,6 +31,8 @@
     ".": {
       "react-server": "./react-dom.react-server.js",
       "default": "./index.js"
-    }
+    },
+    "./src/*": "./src/*",
+    "./package.json": "./package.json"
   }
 }
diff --git a/scripts/rollup/bundles.js b/scripts/rollup/bundles.js
index 66e59124ca2d5..67f1c237b209f 100644
--- a/scripts/rollup/bundles.js
+++ b/scripts/rollup/bundles.js
@@ -363,6 +363,19 @@ const bundles = [
     externals: [],
   },
 
+  /******* React HTML *******/
+  {
+    bundleTypes: [NODE_DEV, NODE_PROD],
+    moduleType: RENDERER,
+    entry: 'react-html/src/ReactHTMLServer.js',
+    name: 'react-html.react-server',
+    condition: 'react-server',
+    global: 'ReactHTML',
+    minifyWithProdErrorCodes: false,
+    wrapWithModuleBoundaries: false,
+    externals: ['react'],
+  },
+
   /******* React Server DOM Webpack Server *******/
   {
     bundleTypes: [NODE_DEV, NODE_PROD],
diff --git a/scripts/rollup/forks.js b/scripts/rollup/forks.js
index a83b8b1b1a969..0ba6827bba7a2 100644
--- a/scripts/rollup/forks.js
+++ b/scripts/rollup/forks.js
@@ -65,6 +65,12 @@ const forks = Object.freeze({
     if (entry === 'react/src/ReactServer.js') {
       return './packages/react/src/ReactSharedInternalsServer.js';
     }
+    if (entry === 'react-html/src/ReactHTMLServer.js') {
+      // Inside the ReactHTMLServer render we don't refer to any shared internals
+      // but instead use our own internal copy of the state because you cannot use
+      // any of this state from a component anyway. E.g. you can't use a client hook.
+      return './packages/react/src/ReactSharedInternalsClient.js';
+    }
     if (bundle.condition === 'react-server') {
       return './packages/react-server/src/ReactSharedInternalsServer.js';
     }
@@ -93,7 +99,8 @@ const forks = Object.freeze({
       entry === 'react-dom' ||
       entry === 'react-dom/src/ReactDOMFB.js' ||
       entry === 'react-dom/src/ReactDOMTestingFB.js' ||
-      entry === 'react-dom/src/ReactDOMServer.js'
+      entry === 'react-dom/src/ReactDOMServer.js' ||
+      entry === 'react-html/src/ReactHTMLServer.js'
     ) {
       if (
         bundleType === FB_WWW_DEV ||
diff --git a/scripts/shared/inlinedHostConfigs.js b/scripts/shared/inlinedHostConfigs.js
index d138e1acc8568..d9c21ad480a4a 100644
--- a/scripts/shared/inlinedHostConfigs.js
+++ b/scripts/shared/inlinedHostConfigs.js
@@ -395,7 +395,7 @@ module.exports = [
     entryPoints: [
       'react-dom/src/server/ReactDOMLegacyServerBrowser.js', // react-dom/server.browser
       'react-dom/src/server/ReactDOMLegacyServerNode.js', // react-dom/server.node
-      'react-html/react-html.react-server.js',
+      'react-html/src/ReactHTMLServer.js',
     ],
     paths: [
       'react-dom',

From 23a7c2dc3d59b84e8576732ac64bb310ccf29a9b Mon Sep 17 00:00:00 2001
From: Sebastian Markbage <sebastian@calyptus.eu>
Date: Thu, 20 Jun 2024 11:50:53 +0200
Subject: [PATCH 4/8] Tests

We cannot use JSX because we need to use the react-server runtime.
---
 .../src/__tests__/ReactHTMLServer-test.js     | 77 ++++++++++++++++++-
 1 file changed, 75 insertions(+), 2 deletions(-)

diff --git a/packages/react-html/src/__tests__/ReactHTMLServer-test.js b/packages/react-html/src/__tests__/ReactHTMLServer-test.js
index 592162663308e..8d74e38f24940 100644
--- a/packages/react-html/src/__tests__/ReactHTMLServer-test.js
+++ b/packages/react-html/src/__tests__/ReactHTMLServer-test.js
@@ -27,10 +27,83 @@ describe('ReactHTML', () => {
 
   it('should be able to render a simple component', async () => {
     function Component() {
-      return <div>hello world</div>;
+      // We can't use JSX because that's client-JSX in our tests.
+      return React.createElement('div', null, 'hello world');
     }
 
-    const html = await ReactHTML.renderToMarkup(<Component />);
+    const html = await ReactHTML.renderToMarkup(React.createElement(Component));
     expect(html).toBe('<div>hello world</div>');
   });
+
+  it('supports the useId Hook', async () => {
+    function Component() {
+      const firstNameId = React.useId();
+      const lastNameId = React.useId();
+      // We can't use JSX because that's client-JSX in our tests.
+      return React.createElement(
+        'div',
+        null,
+        React.createElement(
+          'h2',
+          {
+            id: firstNameId,
+          },
+          'First',
+        ),
+        React.createElement(
+          'p',
+          {
+            'aria-labelledby': firstNameId,
+          },
+          'Sebastian',
+        ),
+        React.createElement(
+          'h2',
+          {
+            id: lastNameId,
+          },
+          'Last',
+        ),
+        React.createElement(
+          'p',
+          {
+            'aria-labelledby': lastNameId,
+          },
+          'Smith',
+        ),
+      );
+    }
+
+    const html = await ReactHTML.renderToMarkup(React.createElement(Component));
+    const container = document.createElement('div');
+    container.innerHTML = html;
+
+    expect(container.getElementsByTagName('h2')[0].id).toBe(
+      container.getElementsByTagName('p')[0].getAttribute('aria-labelledby'),
+    );
+    expect(container.getElementsByTagName('h2')[1].id).toBe(
+      container.getElementsByTagName('p')[1].getAttribute('aria-labelledby'),
+    );
+
+    // It's not the same id between them.
+    expect(container.getElementsByTagName('h2')[0].id).not.toBe(
+      container.getElementsByTagName('p')[1].getAttribute('aria-labelledby'),
+    );
+  });
+
+  // @gate enableCache
+  it('supports cache', async () => {
+    let counter = 0;
+    const getCount = React.cache(() => {
+      return counter++;
+    });
+    function Component() {
+      const a = getCount();
+      const b = getCount();
+      return React.createElement('div', null, a, b);
+    }
+
+    const html = await ReactHTML.renderToMarkup(React.createElement(Component));
+    expect(html).toBe('<div>00</div>');
+  });
 });

From 5a7d35a3894c2ca35f57309e1fdff850424e234b Mon Sep 17 00:00:00 2001
From: Sebastian Markbage <sebastian@calyptus.eu>
Date: Thu, 20 Jun 2024 17:59:23 +0200
Subject: [PATCH 5/8] Configure Flight rendering

We render into an RSC payload using FlightServer, then parse it with
FlightClient and then render the result using Fizz.
---
 .../forks/ReactFlightClientConfig.dom-bun.js  |   1 -
 .../ReactFlightClientConfig.dom-legacy.js     |  92 +++++++++++---
 .../src/ReactHTMLLegacyClientStreamConfig.js  |  32 +++++
 packages/react-html/src/ReactHTMLServer.js    | 115 +++++++++++++++---
 .../src/__tests__/ReactHTMLServer-test.js     |   3 +
 .../ReactFlightServerConfig.dom-legacy.js     |  75 +++++++++++-
 scripts/error-codes/codes.json                |   6 +-
 scripts/shared/inlinedHostConfigs.js          |  33 +++++
 8 files changed, 318 insertions(+), 39 deletions(-)
 create mode 100644 packages/react-html/src/ReactHTMLLegacyClientStreamConfig.js

diff --git a/packages/react-client/src/forks/ReactFlightClientConfig.dom-bun.js b/packages/react-client/src/forks/ReactFlightClientConfig.dom-bun.js
index 461996a2e0887..0a8027e3e12aa 100644
--- a/packages/react-client/src/forks/ReactFlightClientConfig.dom-bun.js
+++ b/packages/react-client/src/forks/ReactFlightClientConfig.dom-bun.js
@@ -11,7 +11,6 @@ export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
 export * from 'react-client/src/ReactClientConsoleConfigPlain';
 export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';
 
-export type Response = any;
 export opaque type ModuleLoading = mixed;
 export opaque type SSRModuleMap = mixed;
 export opaque type ServerManifest = mixed;
diff --git a/packages/react-client/src/forks/ReactFlightClientConfig.dom-legacy.js b/packages/react-client/src/forks/ReactFlightClientConfig.dom-legacy.js
index ddf6440a20e48..8e91e3a8062fd 100644
--- a/packages/react-client/src/forks/ReactFlightClientConfig.dom-legacy.js
+++ b/packages/react-client/src/forks/ReactFlightClientConfig.dom-legacy.js
@@ -7,20 +7,82 @@
  * @flow
  */
 
-export * from 'react-client/src/ReactFlightClientStreamConfigWeb';
-export * from 'react-client/src/ReactClientConsoleConfigBrowser';
-export * from 'react-dom-bindings/src/shared/ReactFlightClientConfigDOM';
-
-export type Response = any;
-export opaque type ModuleLoading = mixed;
-export opaque type SSRModuleMap = mixed;
-export opaque type ServerManifest = mixed;
+import type {Thenable} from 'shared/ReactTypes';
+
+export * from 'react-html/src/ReactHTMLLegacyClientStreamConfig.js';
+export * from 'react-client/src/ReactClientConsoleConfigPlain';
+
+export type ModuleLoading = null;
+export type SSRModuleMap = null;
+export opaque type ServerManifest = null;
 export opaque type ServerReferenceId = string;
-export opaque type ClientReferenceMetadata = mixed;
-export opaque type ClientReference<T> = mixed; // eslint-disable-line no-unused-vars
-export const resolveClientReference: any = null;
-export const resolveServerReference: any = null;
-export const preloadModule: any = null;
-export const requireModule: any = null;
-export const prepareDestinationForModule: any = null;
+export opaque type ClientReferenceMetadata = null;
+export opaque type ClientReference<T> = null; // eslint-disable-line no-unused-vars
+
+export function prepareDestinationForModule(
+  moduleLoading: ModuleLoading,
+  nonce: ?string,
+  metadata: ClientReferenceMetadata,
+) {
+  throw new Error(
+    'renderToMarkup should not have emitted Client References. This is a bug in React.',
+  );
+}
+
+export function resolveClientReference<T>(
+  bundlerConfig: SSRModuleMap,
+  metadata: ClientReferenceMetadata,
+): ClientReference<T> {
+  throw new Error(
+    'renderToMarkup should not have emitted Client References. This is a bug in React.',
+  );
+}
+
+export function resolveServerReference<T>(
+  config: ServerManifest,
+  id: ServerReferenceId,
+): ClientReference<T> {
+  throw new Error(
+    'renderToMarkup should not have emitted Server References. This is a bug in React.',
+  );
+}
+
+export function preloadModule<T>(
+  metadata: ClientReference<T>,
+): null | Thenable<T> {
+  return null;
+}
+
+export function requireModule<T>(metadata: ClientReference<T>): T {
+  throw new Error(
+    'renderToMarkup should not have emitted Client References. This is a bug in React.',
+  );
+}
+
 export const usedWithSSR = true;
+
+type HintCode = string;
+type HintModel<T: HintCode> = null; // eslint-disable-line no-unused-vars
+
+export function dispatchHint<Code: HintCode>(
+  code: Code,
+  model: HintModel<Code>,
+): void {
+  // Should never happen.
+}
+
+export function preinitModuleForSSR(
+  href: string,
+  nonce: ?string,
+  crossOrigin: ?string,
+) {
+  // Should never happen.
+}
+
+export function preinitScriptForSSR(
+  href: string,
+  nonce: ?string,
+  crossOrigin: ?string,
+) {
+  // Should never happen.
+}
diff --git a/packages/react-html/src/ReactHTMLLegacyClientStreamConfig.js b/packages/react-html/src/ReactHTMLLegacyClientStreamConfig.js
new file mode 100644
index 0000000000000..74b0503590462
--- /dev/null
+++ b/packages/react-html/src/ReactHTMLLegacyClientStreamConfig.js
@@ -0,0 +1,32 @@
+/**
+ * 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.
+ *
+ * @flow
+ */
+
+// TODO: The legacy one should not use binary.
+
+export type StringDecoder = TextDecoder;
+
+export function createStringDecoder(): StringDecoder {
+  return new TextDecoder();
+}
+
+const decoderOptions = {stream: true};
+
+export function readPartialStringChunk(
+  decoder: StringDecoder,
+  buffer: Uint8Array,
+): string {
+  return decoder.decode(buffer, decoderOptions);
+}
+
+export function readFinalStringChunk(
+  decoder: StringDecoder,
+  buffer: Uint8Array,
+): string {
+  return decoder.decode(buffer);
+}
diff --git a/packages/react-html/src/ReactHTMLServer.js b/packages/react-html/src/ReactHTMLServer.js
index 440f7ae337d0e..c4eebe3f51054 100644
--- a/packages/react-html/src/ReactHTMLServer.js
+++ b/packages/react-html/src/ReactHTMLServer.js
@@ -8,20 +8,29 @@
  */
 
 import type {ReactNodeList} from 'shared/ReactTypes';
-
-import type {
-  Request,
-  PostponedState,
-  ErrorInfo,
-} from 'react-server/src/ReactFizzServer';
+import type {LazyComponent} from 'react/src/ReactLazy';
 
 import ReactVersion from 'shared/ReactVersion';
 
 import {
-  createRequest,
-  startWork,
-  startFlowing,
-  abort,
+  createRequest as createFlightRequest,
+  startWork as startFlightWork,
+  startFlowing as startFlightFlowing,
+  abort as abortFlight,
+} from 'react-server/src/ReactFlightServer';
+
+import {
+  createResponse as createFlightResponse,
+  getRoot as getFlightRoot,
+  processBinaryChunk as processFlightBinaryChunk,
+  close as closeFlight,
+} from 'react-client/src/ReactFlightClient';
+
+import {
+  createRequest as createFizzRequest,
+  startWork as startFizzWork,
+  startFlowing as startFizzFlowing,
+  abort as abortFizz,
 } from 'react-server/src/ReactFizzServer';
 
 import {
@@ -30,20 +39,60 @@ import {
   createRootFormatContext,
 } from 'react-dom-bindings/src/server/ReactFizzConfigDOMLegacy';
 
+type ReactMarkupNodeList =
+  // This is the intersection of ReactNodeList and ReactClientValue minus
+  // Client/ServerReferences.
+  | React$Element<React$AbstractComponent<any, any>>
+  | LazyComponent<ReactMarkupNodeList, any>
+  | React$Element<string>
+  | string
+  | boolean
+  | number
+  | symbol
+  | null
+  | void
+  | bigint
+  | $AsyncIterable<ReactMarkupNodeList, ReactMarkupNodeList, void>
+  | $AsyncIterator<ReactMarkupNodeList, ReactMarkupNodeList, void>
+  | Iterable<ReactMarkupNodeList>
+  | Iterator<ReactMarkupNodeList>
+  | Array<ReactMarkupNodeList>
+  | Promise<ReactMarkupNodeList>; // Thenable<ReactMarkupNodeList>
+
 type MarkupOptions = {
   identifierPrefix?: string,
   signal?: AbortSignal,
 };
 
+function noServerCallOrFormAction() {
+  throw new Error(
+    'renderToMarkup should not have emitted Server References. This is a bug in React.',
+  );
+}
+
 export function renderToMarkup(
-  children: ReactNodeList,
+  children: ReactMarkupNodeList,
   options?: MarkupOptions,
 ): Promise<string> {
   return new Promise((resolve, reject) => {
-    let didFatal = false;
-    let fatalError = null;
+    const textEncoder = new TextEncoder();
+    const flightDestination = {
+      push(chunk: string | null): boolean {
+        if (chunk !== null) {
+          // TODO: Legacy should not use binary streams.
+          processFlightBinaryChunk(flightResponse, textEncoder.encode(chunk));
+        } else {
+          closeFlight(flightResponse);
+        }
+        return true;
+      },
+      destroy(error: mixed): void {
+        abortFizz(fizzRequest, error);
+        reject(error);
+      },
+    };
     let buffer = '';
-    const destination = {
+    const fizzDestination = {
       // $FlowFixMe[missing-local-annot]
       push(chunk) {
         if (chunk !== null) {
@@ -56,6 +105,7 @@ export function renderToMarkup(
       },
       // $FlowFixMe[missing-local-annot]
       destroy(error) {
+        abortFlight(flightRequest, error);
         reject(error);
       },
     };
@@ -65,12 +115,33 @@ export function renderToMarkup(
       // client rendering mode because there's no client rendering here.
       reject(error);
     }
+    const flightRequest = createFlightRequest(
+      // $FlowFixMe: This should be a subtype but not everything is typed covariant.
+      children,
+      null,
+      onError,
+      options ? options.identifierPrefix : undefined,
+      undefined,
+      'Markup',
+      undefined,
+    );
+    const flightResponse = createFlightResponse(
+      null,
+      null,
+      noServerCallOrFormAction,
+      noServerCallOrFormAction,
+      undefined,
+      undefined,
+      undefined,
+    );
     const resumableState = createResumableState(
       options ? options.identifierPrefix : undefined,
       undefined,
     );
-    const request = createRequest(
-      children,
+    const root = getFlightRoot<ReactNodeList>(flightResponse);
+    const fizzRequest = createFizzRequest(
+      // $FlowFixMe: Thenables as children are supported.
+      root,
       resumableState,
       createRenderState(resumableState, true),
       createRootFormatContext(),
@@ -86,17 +157,21 @@ export function renderToMarkup(
     if (options && options.signal) {
       const signal = options.signal;
       if (signal.aborted) {
-        abort(request, (signal: any).reason);
+        abortFlight(flightRequest, (signal: any).reason);
+        abortFizz(fizzRequest, (signal: any).reason);
       } else {
         const listener = () => {
-          abort(request, (signal: any).reason);
+          abortFlight(flightRequest, (signal: any).reason);
+          abortFizz(fizzRequest, (signal: any).reason);
           signal.removeEventListener('abort', listener);
         };
         signal.addEventListener('abort', listener);
       }
     }
-    startWork(request);
-    startFlowing(request, destination);
+    startFlightWork(flightRequest);
+    startFlightFlowing(flightRequest, flightDestination);
+    startFizzWork(fizzRequest);
+    startFizzFlowing(fizzRequest, fizzDestination);
   });
 }
 
diff --git a/packages/react-html/src/__tests__/ReactHTMLServer-test.js b/packages/react-html/src/__tests__/ReactHTMLServer-test.js
index 8d74e38f24940..503e233301c9a 100644
--- a/packages/react-html/src/__tests__/ReactHTMLServer-test.js
+++ b/packages/react-html/src/__tests__/ReactHTMLServer-test.js
@@ -9,6 +9,9 @@
 
 'use strict';
 
+global.TextDecoder = require('util').TextDecoder;
+global.TextEncoder = require('util').TextEncoder;
+
 let React;
 let ReactHTML;
 
diff --git a/packages/react-server/src/forks/ReactFlightServerConfig.dom-legacy.js b/packages/react-server/src/forks/ReactFlightServerConfig.dom-legacy.js
index bfe794c9a2c55..0c0d5113d994c 100644
--- a/packages/react-server/src/forks/ReactFlightServerConfig.dom-legacy.js
+++ b/packages/react-server/src/forks/ReactFlightServerConfig.dom-legacy.js
@@ -9,9 +9,19 @@
 
 import type {Request} from 'react-server/src/ReactFlightServer';
 import type {ReactComponentInfo} from 'shared/ReactTypes';
+import type {ReactClientValue} from 'react-server/src/ReactFlightServer';
 
-export * from '../ReactFlightServerConfigBundlerCustom';
-export * from 'react-dom-bindings/src/server/ReactFlightServerConfigDOM';
+// Used to distinguish these contexts from ones used in other renderers.
+// E.g. this can be used to distinguish legacy renderers from this modern one.
+export const isPrimaryRenderer = true;
+
+export type HintCode = string;
+export type HintModel<T: HintCode> = null; // eslint-disable-line no-unused-vars
+export type Hints = null;
+
+export function createHints(): Hints {
+  return null;
+}
 
 export const supportsRequestStorage = false;
 export const requestStorage: AsyncLocalStorage<Request | void> = (null: any);
@@ -21,3 +31,64 @@ export const componentStorage: AsyncLocalStorage<ReactComponentInfo | void> =
   (null: any);
 
 export * from '../ReactFlightServerConfigDebugNoop';
+
+export type ClientManifest = null;
+export opaque type ClientReference<T> = null; // eslint-disable-line no-unused-vars
+export opaque type ServerReference<T> = null; // eslint-disable-line no-unused-vars
+export opaque type ClientReferenceMetadata: any = null;
+export opaque type ServerReferenceId: string = string;
+export opaque type ClientReferenceKey: any = string;
+
+const CLIENT_REFERENCE_TAG = Symbol.for('react.client.reference');
+const SERVER_REFERENCE_TAG = Symbol.for('react.server.reference');
+
+export function isClientReference(reference: Object): boolean {
+  return reference.$$typeof === CLIENT_REFERENCE_TAG;
+}
+
+export function isServerReference(reference: Object): boolean {
+  return reference.$$typeof === SERVER_REFERENCE_TAG;
+}
+
+export function getClientReferenceKey(
+  reference: ClientReference<any>,
+): ClientReferenceKey {
+  throw new Error(
+    'Attempted to render a Client Component from renderToMarkup. ' +
+      'This is not supported since it will never hydrate. ' +
+      'Only render Server Components with renderToMarkup.',
+  );
+}
+
+export function resolveClientReferenceMetadata<T>(
+  config: ClientManifest,
+  clientReference: ClientReference<T>,
+): ClientReferenceMetadata {
+  throw new Error(
+    'Attempted to render a Client Component from renderToMarkup. ' +
+      'This is not supported since it will never hydrate. ' +
+      'Only render Server Components with renderToMarkup.',
+  );
+}
+
+export function getServerReferenceId<T>(
+  config: ClientManifest,
+  serverReference: ServerReference<T>,
+): ServerReferenceId {
+  throw new Error(
+    'Attempted to render a Server Action from renderToMarkup. ' +
+      'This is not supported since it varies by version of the app. ' +
+      'Use a fixed URL for any forms instead.',
+  );
+}
+
+export function getServerReferenceBoundArguments<T>(
+  config: ClientManifest,
+  serverReference: ServerReference<T>,
+): null | Array<ReactClientValue> {
+  throw new Error(
+    'Attempted to render a Server Action from renderToMarkup. ' +
+      'This is not supported since it varies by version of the app. ' +
+      'Use a fixed URL for any forms instead.',
+  );
+}
diff --git a/scripts/error-codes/codes.json b/scripts/error-codes/codes.json
index 63c0e7061e882..9d8fb4ed3739f 100644
--- a/scripts/error-codes/codes.json
+++ b/scripts/error-codes/codes.json
@@ -516,5 +516,9 @@
   "528": "Expected <link> not to update to be updated to a stylesheet with precedence. Check the `rel`, `href`, and `precedence` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.%s",
   "529": "Expected stylesheet with precedence to not be updated to a different kind of <link>. Check the `rel`, `href`, and `precedence` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.%s",
   "530": "The render was aborted by the server with a promise.",
-  "531": "react-html is not supported outside a React Server Components environment."
+  "531": "react-html is not supported outside a React Server Components environment.",
+  "532": "Attempted to render a Client Component from renderToMarkup. This is not supported since it will never hydrate. Only render Server Components with renderToMarkup.",
+  "533": "Attempted to render a Server Action from renderToMarkup. This is not supported since it varies by version of the app. Use a fixed URL for any forms instead.",
+  "534": "renderToMarkup should not have emitted Client References. This is a bug in React.",
+  "535": "renderToMarkup should not have emitted Server References. This is a bug in React."
 }
diff --git a/scripts/shared/inlinedHostConfigs.js b/scripts/shared/inlinedHostConfigs.js
index d9c21ad480a4a..a21b624150aac 100644
--- a/scripts/shared/inlinedHostConfigs.js
+++ b/scripts/shared/inlinedHostConfigs.js
@@ -36,6 +36,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.node',
       'react-dom/src/server/ReactDOMFizzServerNode.js', // react-dom/server.node
       'react-dom/src/server/ReactDOMFizzStaticNode.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-webpack',
       'react-server-dom-webpack/client.node.unbundled',
       'react-server-dom-webpack/server',
@@ -71,6 +74,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.node',
       'react-dom/src/server/ReactDOMFizzServerNode.js', // react-dom/server.node
       'react-dom/src/server/ReactDOMFizzStaticNode.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-webpack',
       'react-server-dom-webpack/client.node',
       'react-server-dom-webpack/server',
@@ -108,6 +114,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.node',
       'react-dom/src/server/ReactDOMFizzServerNode.js', // react-dom/server.node
       'react-dom/src/server/ReactDOMFizzStaticNode.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-turbopack',
       'react-server-dom-turbopack/client.node.unbundled',
       'react-server-dom-turbopack/server',
@@ -145,6 +154,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.node',
       'react-dom/src/server/ReactDOMFizzServerNode.js', // react-dom/server.node
       'react-dom/src/server/ReactDOMFizzStaticNode.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-turbopack',
       'react-server-dom-turbopack/client.node',
       'react-server-dom-turbopack/server',
@@ -182,6 +194,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.bun',
       'react-dom/src/server/ReactDOMFizzServerBun.js',
       'react-dom-bindings',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'shared/ReactDOMSharedInternals',
     ],
     isFlowTyped: true,
@@ -212,6 +227,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.browser',
       'react-dom/src/server/ReactDOMFizzServerBrowser.js', // react-dom/server.browser
       'react-dom/src/server/ReactDOMFizzStaticBrowser.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-webpack',
       'react-server-dom-webpack/client',
       'react-server-dom-webpack/client.browser',
@@ -240,6 +258,9 @@ module.exports = [
       'react-dom/server',
       'react-dom/server.node',
       'react-dom-bindings',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-esm',
       'react-server-dom-esm/client',
       'react-server-dom-esm/client.browser',
@@ -274,6 +295,9 @@ module.exports = [
       'react-server-dom-turbopack/src/ReactFlightDOMServerBrowser.js', // react-server-dom-turbopack/server.browser
       'react-server-dom-turbopack/src/ReactFlightClientConfigBundlerTurbopack.js',
       'react-server-dom-turbopack/src/ReactFlightClientConfigBundlerTurbopackBrowser.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-devtools',
       'react-devtools-core',
       'react-devtools-shell',
@@ -303,6 +327,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.edge',
       'react-dom/src/server/ReactDOMFizzServerEdge.js', // react-dom/server.edge
       'react-dom/src/server/ReactDOMFizzStaticEdge.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-webpack',
       'react-server-dom-webpack/client.edge',
       'react-server-dom-webpack/server.edge',
@@ -338,6 +365,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.edge',
       'react-dom/src/server/ReactDOMFizzServerEdge.js', // react-dom/server.edge
       'react-dom/src/server/ReactDOMFizzStaticEdge.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-turbopack',
       'react-server-dom-turbopack/client.edge',
       'react-server-dom-turbopack/server.edge',
@@ -374,6 +404,9 @@ module.exports = [
       'react-dom/src/server/react-dom-server.node',
       'react-dom/src/server/ReactDOMFizzServerNode.js', // react-dom/server.node
       'react-dom/src/server/ReactDOMFizzStaticNode.js',
+      'react-dom-bindings/src/server/ReactDOMFlightServerHostDispatcher.js',
+      'react-dom-bindings/src/server/ReactFlightServerConfigDOM.js',
+      'react-dom-bindings/src/shared/ReactFlightClientConfigDOM.js',
       'react-server-dom-esm',
       'react-server-dom-esm/client.node',
       'react-server-dom-esm/server',

From 2829604ebd1bbd25ab14d4a89a440822aa31f536 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Markb=C3=A5ge?= <sebastian@calyptus.eu>
Date: Thu, 27 Jun 2024 09:47:48 -0400
Subject: [PATCH 6/8] Update packages/react-html/package.json

Typo for npm. This isn't covered by tests since our tests manually alias instead of looking at the conditions.

Co-authored-by: Sebastian Silbermann <silbermann.sebastian@gmail.com>
---
 packages/react-html/package.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/react-html/package.json b/packages/react-html/package.json
index c9c4205304b53..92dfc38512338 100644
--- a/packages/react-html/package.json
+++ b/packages/react-html/package.json
@@ -29,7 +29,7 @@
   ],
   "exports": {
     ".": {
-      "react-server": "./react-dom.react-server.js",
+      "react-server": "./react-html.react-server.js",
       "default": "./index.js"
     },
     "./src/*": "./src/*",

From 7817ea558c16fb37273eb025b6271bd253b85f22 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Markb=C3=A5ge?= <sebastian@calyptus.eu>
Date: Thu, 27 Jun 2024 09:48:01 -0400
Subject: [PATCH 7/8] Update README

Co-authored-by: Sebastian Silbermann <silbermann.sebastian@gmail.com>
---
 packages/react-html/README.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/react-html/README.md b/packages/react-html/README.md
index a4acd1fd84396..c0794aac104c5 100644
--- a/packages/react-html/README.md
+++ b/packages/react-html/README.md
@@ -1,6 +1,6 @@
 # `react-html`
 
-This package provides the ability to render standalone HTML from Server Components for use in embedded contexts such as e-mails and RSS/Atom feeds. It cannot use Client Components and aren't hydrated. It is intended to be paired with the generic React package, which is shipped as `react` to npm.
+This package provides the ability to render standalone HTML from Server Components for use in embedded contexts such as e-mails and RSS/Atom feeds. It cannot use Client Components and does not hydrate. It is intended to be paired with the generic React package, which is shipped as `react` to npm.
 
 ## Installation
 
@@ -23,7 +23,7 @@ async function action(email, name) {
 }
 ```
 
-Note that this is an async function that need to be awaited - unlike the legacy `renderToString` in `react-dom`.
+Note that this is an async function that needs to be awaited - unlike the legacy `renderToString` in `react-dom`.
 
 ## API
 

From 471c13ba1c3a98c5b741f1803d0959d170a66a64 Mon Sep 17 00:00:00 2001
From: Sebastian Markbage <sebastian@calyptus.eu>
Date: Thu, 27 Jun 2024 09:59:09 -0400
Subject: [PATCH 8/8] Remove isPrimaryRenderer

No longer used since there's no Context on the Server.
---
 .../src/forks/ReactFlightServerConfig.dom-legacy.js           | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/packages/react-server/src/forks/ReactFlightServerConfig.dom-legacy.js b/packages/react-server/src/forks/ReactFlightServerConfig.dom-legacy.js
index 0c0d5113d994c..99591bb954ea9 100644
--- a/packages/react-server/src/forks/ReactFlightServerConfig.dom-legacy.js
+++ b/packages/react-server/src/forks/ReactFlightServerConfig.dom-legacy.js
@@ -11,10 +11,6 @@ import type {Request} from 'react-server/src/ReactFlightServer';
 import type {ReactComponentInfo} from 'shared/ReactTypes';
 import type {ReactClientValue} from 'react-server/src/ReactFlightServer';
 
-// Used to distinguish these contexts from ones used in other renderers.
-// E.g. this can be used to distinguish legacy renderers from this modern one.
-export const isPrimaryRenderer = true;
-
 export type HintCode = string;
 export type HintModel<T: HintCode> = null; // eslint-disable-line no-unused-vars
 export type Hints = null;