css/cssom/CSSStyleSheet-constructable-cssRules.html

<!doctype html>
<meta charset="utf-8">
<title>CSSStyleSheet.replace/replaceSync() doesn't change cssRules object</title>
<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
<link rel="author" href="https://mozilla.org" title="Mozilla">
<link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1752392">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<span>Should be green</span>
<script>
promise_test(async function() {
  const sheet = new CSSStyleSheet();
  let rules = sheet.cssRules;
  sheet.replaceSync('span {color:blue;}');
  assert_equals(rules, sheet.cssRules, "Rules should be the same after replaceSync");
  await sheet.replace('span {color: lime;}');
  assert_equals(rules, sheet.cssRules, "Rules should be the same after replace()");
  document.adoptedStyleSheets = [sheet];
  assert_equals(getComputedStyle(document.querySelector("span")).color, "rgb(0, 255, 0)", "Sheet should apply");
}, "cssRules doesn't change on replace / replaceSync");
</script>