chromium/third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/claim-worker-fetch-iframe.html

<!doctype html>
<script>
var worker = new Worker('./claim-worker-fetch-worker.js');

function fetch_in_worker(url) {
  return new Promise((resolve) => {
    worker.onmessage = (event) => {
      resolve(event.data);
    };
    worker.postMessage(url);
  });
}
</script>