firefox/testing/web-platform/tests/permissions-policy/resources/permissions-policy-idle-detection-worker.html

<script>
'use strict';

let worker = new Worker('permissions-policy-idle-detection-worker.js');

worker.onmessage = event => {
  window.parent.postMessage(event.data, '*');
};
worker.postMessage({ type: 'ready' });
</script>