Codebase Browser
chromium
Go to App

chromium/third_party/blink/web_tests/external/wpt/speculation-rules/prerender/resources/post-message-prerendering-completion-notification.html 

<!doctype html>
<body>
<script>
window.parent.postMessage(`document.prerendering: ${document.prerendering}`,
                          '*');
</script>
</body>