Codebase Browser
chromium
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chromium/third_party/blink/web_tests/external/wpt/html/browsers/browsing-the-web/resources/has-iframe.html 

<!DOCTYPE html>
<iframe src="/common/blank.html"></iframe>
<script>
window.onload = () => {
  window.opener.postMessage("top ready", "*");
};
</script>