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chromium/third_party/blink/web_tests/external/wpt/css/cssom/insertRule-across-context.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Test: CSSOM StyleSheet insertRule across context</title>
<link rel="author" title="Kagami Sascha Rosylight" href="mailto:[email protected]">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="help" href="https://drafts.csswg.org/cssom/">
<script src="/resources/testharness.js" type="text/javascript"></script>
<script src="/resources/testharnessreport.js" type="text/javascript"></script>

<iframe id="iframe"></iframe>
<script>
  function createSheet() {
    const tempStyleElement = iframe.contentDocument.createElement('style');
    iframe.contentDocument.head.append(tempStyleElement);

    const tempStyle = tempStyleElement.sheet;
    tempStyleElement.remove();
    return tempStyle;
  }

  test(() => {
    const sheet = createSheet();
    sheet.insertRule(".kaoru {}");
    assert_equals(sheet.rules[0].constructor, iframe.contentWindow.CSSStyleRule);
  }, "The constructor of inserted rule object must be from iframe");

  test(() => {
    const sheet = new iframe.contentWindow.CSSStyleSheet();
    sheet.insertRule(".kaoru {}");
    assert_equals(sheet.rules[0].constructor, iframe.contentWindow.CSSStyleRule);
  }, "The constructor of inserted rule object must be from iframe for new CSSStyleSheet()");
</script>