Service Worker gotchas – https://medium.com/kollegorna/service-worker-gotchas-af20a9dab986

service-worker-online-assets-offline-html.js

self.addEventListener('fetch', event => {
  var type = event.request.headers.get('Accept');
  // network-first for HTML documents
  if(type.includes('text/html')) {
    event.respondWith(
      // respondWith code from network-first section
    );
  }
  // cache-first for assets
  else {
    event.respondWith(
      // respondWith code from cache-first section
    );
  }
});