Background Fetch. Step 1: Doing something...

index.html

<button id="bgFetchButton">Store assets locally</button>
<script>
  bgFetchButton = document.querySelector('#bgFetchButton');

  bgFetchButton.addEventListener('click', async event => {
    try {
      const registration = await navigator.serviceWorker.ready;
      registration.backgroundFetch.fetch('my-fetch', ['/assets/s01e01.mpg']);
    } catch (err) {
      console.error(err);
    }
  });

  if ('serviceWorker' in navigator) {
    window.addEventListener('load', () => {
      navigator.serviceWorker.register('/service-worker.js');
    });
  }
</script>

service-worker.js

addEventListener('backgroundfetchsuccess', event => {
  console.log('[Service Worker]: Background Fetch Success', event.registration);
});

self.addEventListener('install', event => {
  console.log('[Service Worker]: Installed');
});

self.addEventListener('activate', event => {
  console.log('[Service Worker]: Active');
});