index.html

<!DOCTYPE html>
<html>
  <head>
    <title>GistRun</title>
    <script>
      'use strict'

      const targetOrigin = location.origin === 'https://gist.host' ? 'https://gist.run' : 'http://localhost:9000';

      navigator.serviceWorker.register(`${location.origin}/worker.js`, { scope: './' });

      addEventListener('message', event => {
        if (event.origin !== targetOrigin) {
          return;
        }
        navigator.serviceWorker.ready.then(registration => {
          registration.active.postMessage(event.data, event.ports);
        });
      });

      setTimeout(function() { parent.postMessage('worker page ready', targetOrigin); });
    </script>
  </head>
  <body>
    Worker Activator
  </body>
</html>