sw-04-fetching.js

function isHtmlRequest(request) {
  return (
    request.method === 'GET' &&
    request.headers.get('accept').includes('text/html')
  );
}

self.addEventListener('fetch', event => {
  if (isHtmlRequest(event.request)) {
    event.respondWith(
      fetch(event.request).catch(() => {
        return caches.match(OFFLINE_URL);
      })
    );
  }
});