test/e2e/resources/network/service-worker.html

<div id="content"></div>
<script>
  navigator.serviceWorker.register('./service-worker.js');
  navigator.serviceWorker.addEventListener('message', event => {
    document.getElementById('content').innerText = event.data;
  });

  navigator.serviceWorker.ready.then(registration => {
    registration.active.postMessage('ping');
  });
</script>