Codebase Browser
chromium
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chromium/third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/nested-iframe-parent.html 

<!DOCTYPE html>
<script>
  navigator.serviceWorker.onmessage = event => parent.postMessage(event.data, '*', event.ports);
</script>
<iframe id='child'></iframe>