test/e2e/resources/elements/adopted-stylesheet-iframe.html

<div class="from-iframe">from iframe</div>
<script>
  const iframeStyle = new CSSStyleSheet();
  iframeStyle.replaceSync('/* For iframe */\n.from-iframe {\n  color: blue;\n}\n');
  document.adoptedStyleSheets = [iframeStyle];
</script>