chromium/chrome/test/data/background_fetch/background_fetch_iframe.html

<!doctype html>
<html>
  <head>
    <meta charset="utf-8" />
    <script src="../result_queue.js"></script>
  </head>
  <body>
    <script>
      navigator.serviceWorker.ready.then(swRegistration => {
        return swRegistration.backgroundFetch.fetch(
            'iframe-fetch', '/background_fetch/types_of_cheese.txt');
      }).catch(e => {
        window.parent.postMessage('permissionerror', '*');
      });
    </script>
  </body>
</html>