[Fizz] escape <style> textContent as css

style text content has historically been escaped as HTML which is non-sensical and often leads users to using dangerouslySetInnerHTML as a matter of course. While rendering untrusted style rules is a security risk React doesn't really provide any special protection here and forcing users to use a completely unescaped API is if anything worse. So this PR updates the style escaping rules for Fizz to only escape the text content to ensure the tag scope cannot be closed early. This is accomplished by encoding "s" and "S" as hexadecimal unicode representation "\73 " and "\53 " respectively when found within a sequence like </style>. We have to be careful to support casing here just like with the script closing tag regex for bootstrap scripts.

packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js

@@ -2669,6 +2669,26 @@ function pushStyle(
   }
 }
 
+/**
+ * This escaping function is designed to work with style tag textContent only.
+ *
+ * While untrusted style content should be made safe before using this api it will
+ * ensure that the style cannot be early terminated or never terminated state
+ */
+function escapeStyleTextContent(styleText: string) {
+  if (__DEV__) {
+    checkHtmlStringCoercion(styleText);
+  }
+  return ('' + styleText).replace(styleRegex, styleReplacer);
+}
+const styleRegex = /(<\/|<)(s)(tyle)/gi;
+const styleReplacer = (
+  match: string,
+  prefix: string,
+  s: string,
+  suffix: string,
+) => `${prefix}${s === 's' ? '\\73 ' : '\\53 '}${suffix}`;
+
 function pushStyleImpl(
   target: Array<Chunk | PrecomputedChunk>,
   props: Object,
@@ -2710,7 +2730,7 @@ function pushStyleImpl(
     child !== undefined
   ) {
     // eslint-disable-next-line react-internal/safe-string-coercion
-    target.push(stringToChunk(escapeTextForBrowser('' + child)));
+    target.push(stringToChunk(escapeStyleTextContent(child)));
   }
   pushInnerHTML(target, innerHTML, children);
   target.push(endChunkForTag('style'));
@@ -2752,7 +2772,7 @@ function pushStyleContents(
     child !== undefined
   ) {
     // eslint-disable-next-line react-internal/safe-string-coercion
-    target.push(stringToChunk(escapeTextForBrowser('' + child)));
+    target.push(stringToChunk(escapeStyleTextContent(child)));
   }
   pushInnerHTML(target, innerHTML, children);
   return;

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

@@ -4256,6 +4256,56 @@ describe('ReactDOMFizzServer', () => {
     });
   });
 
+  describe('<style> textContent escaping', () => {
+    it('the "S" in "</?[Ss]style" strings are replaced with unicode escaped lowercase s or S depending on case, preserving case sensitivity of nearby characters', async () => {
+      await act(() => {
+        const {pipe} = renderToPipeableStream(
+          <style>{`
+            .foo::after {
+              content: 'sSsS</style></Style></StYlE><style><Style>sSsS'
+            }
+            body {
+              background-color: blue;
+            }
+          `}</style>,
+        );
+        pipe(writable);
+      });
+      expect(window.getComputedStyle(document.body).backgroundColor).toMatch(
+        'blue',
+      );
+    });
+
+    it('the "S" in "</?[Ss]style" strings are replaced with unicode escaped lowercase s or S depending on case, preserving case sensitivity of nearby characters inside hoistable style tags', async () => {
+      await act(() => {
+        const {pipe} = renderToPipeableStream(
+          <>
+            <style href="foo" precedence="default">{`
+            .foo::after {
+              content: 'sSsS</style></Style></StYlE><style><Style>sSsS'
+            }
+            body {
+              background-color: blue;
+            }
+          `}</style>
+            <style href="bar" precedence="default">{`
+          .foo::after {
+            content: 'sSsS</style></Style></StYlE><style><Style>sSsS'
+          }
+          body {
+            background-color: red;
+          }
+        `}</style>
+          </>,
+        );
+        pipe(writable);
+      });
+      expect(window.getComputedStyle(document.body).backgroundColor).toMatch(
+        'red',
+      );
+    });
+  });
+
   // @gate enableFizzExternalRuntime
   it('supports option to load runtime as an external script', async () => {
     await act(() => {