Codebase Browser
chromium
Go to App

chromium/third_party/blink/web_tests/external/wpt/html/infrastructure/urls/base-url/resources/send-back-base-url.html 

<!DOCTYPE html>
<script>
  addEventListener('message', (event) => {
    if (event.data == 'report baseURI')
      event.source.postMessage(document.baseURI, event.origin);
  });
  parent.postMessage('loaded', '*');
</script>