Don't serialize all cssRules if multiple text nodes exists

packages/rrweb-snapshot/src/snapshot.ts

@@ -632,7 +632,12 @@ function serializeNode(
       if (isStyle && textContent) {
         try {
           // try to read style sheet
-          if ((n.parentNode as HTMLStyleElement).sheet?.cssRules) {
+          if (n.nextSibling || n.previousSibling) {
+            // This is not the only child of the stylesheet.
+            // We can't read all of the sheet's .cssRules and expect them
+            // to _only_ include the current rule(s) added by the text node.
+            // So we'll be conservative and keep textContent as-is.
+          } else if ((n.parentNode as HTMLStyleElement).sheet?.cssRules) {
             textContent = stringifyStyleSheet(
               (n.parentNode as HTMLStyleElement).sheet!,
             );

packages/rrweb-snapshot/test/snapshot.test.ts

@@ -1,5 +1,13 @@
+/**
+ * @jest-environment jsdom
+ */
 import { JSDOM } from 'jsdom';
-import { absoluteToStylesheet, _isBlockedElement } from '../src/snapshot';
+import {
+  absoluteToStylesheet,
+  serializeNodeWithId,
+  _isBlockedElement,
+} from '../src/snapshot';
+import { serializedNodeWithId } from '../src/types';
 
 describe('absolute url to stylesheet', () => {
   const href = 'http://localhost/css/style.css';
@@ -126,3 +134,48 @@ describe('isBlockedElement()', () => {
     ).toEqual(true);
   });
 });
+
+describe('style elements', () => {
+  const serializeNode = (node: Node): serializedNodeWithId | null => {
+    return serializeNodeWithId(node, {
+      doc: document,
+      map: {},
+      blockClass: 'blockblock',
+      blockSelector: null,
+      maskTextClass: 'maskmask',
+      maskTextSelector: null,
+      skipChild: false,
+      inlineStylesheet: true,
+      maskTextFn: undefined,
+      maskInputFn: undefined,
+      slimDOMOptions: {},
+    });
+  };
+
+  const render = (html: string): HTMLStyleElement => {
+    document.write(html);
+    return document.querySelector('style')!;
+  };
+
+  it('should serialize all rules of stylesheet when the sheet has a single child node', () => {
+    const styleEl = render(`<style>body { color: red; }</style>`);
+    styleEl.sheet?.insertRule('section { color: blue; }');
+    expect(serializeNode(styleEl.childNodes[0])).toMatchObject({
+      isStyle: true,
+      rootId: undefined,
+      textContent: 'section {color: blue;}body {color: red;}',
+      type: 3,
+    });
+  });
+
+  it('should serialize individual text nodes on stylesheets with multiple child nodes', () => {
+    const styleEl = render(`<style>body { color: red; }</style>`);
+    styleEl.append(document.createTextNode('section { color: blue; }'));
+    expect(serializeNode(styleEl.childNodes[1])).toMatchObject({
+      isStyle: true,
+      rootId: undefined,
+      textContent: 'section { color: blue; }',
+      type: 3,
+    });
+  });
+});