page.js

onPageUnableToLoad(url => {
  tellUserItsBeingFetchedInBackground();
  navigator.serviceWorker.ready.then(reg => {
    reg.sync.register({ tag: 'fetch-n-cache-' + url });
  });
});

sw.js

// serviceWorker
self.addEventListener('sync', event => {
  if (event.registration.tag.startsWith('fetch-n-cache-')) {
    const urlToFetch = event.registration.tag.slice('fetch-n-cache-'.length);

    event.waitUntil(
      caches.open('cached-pages')
        .then(c => c.match(urlToFetch))
        .then(response => {
          if (response) return;
          return c.add(urlToFetch).then(_ => showNotification())
        })
    );
  }
});