chromium/src/third_party/blink/web_tests/http/tests/inspector-protocol/service-worker/resources/periodicsync-event-worker.js

self.addEventListener('periodicsync', async event => {
  const clients = await self.clients.matchAll({ includeUncontrolled: true });
  for (const client of clients)
    client.postMessage('test-tag-from-devtools');
});