chromium/src/third_party/blink/web_tests/http/tests/inspector-protocol/page/resources/iframe-src-write.html

<iframe id="frame" src="http://devtools.test:8000/inspector-protocol/resources/empty.html"></iframe>
<script>
  const doc = document.getElementById('frame').contentDocument;
  doc.open();
  doc.write("<h1>Hello world!</h1>");
  doc.close();
</script>