Basic service worker to cache only selected paths from the installed domain and serve them from cache in case of server error.

sw.js

let version = 1;
let cacheName = self.location.hostname + version;
let allowedPaths = [
  /^\/(\?utm_source=homescreen)?$/,
  // Also matches /article/news/... !!
  /\/news\/?.*$/
];

self.addEventListener('install', (e) => {
  e.waitUntil(
    caches.open(cacheName).then((cache) => {
      return cache.addAll([
        '/',
        '/?utm_source=homescreen'
      ]);
    })
  );
});

self.addEventListener('activate', (e) => {
  if (self.clients && clients.claim) {
    clients.claim();
  }
});

self.addEventListener('fetch', (e) => {
  const requestUrl = new URL(e.request.url);
  if (requestUrl.hostname === self.location.hostname
    && allowedPaths.some((allowedExp) => allowedExp.test(requestUrl.pathname))
  ) {
    e.respondWith(
      fetch(e.request).then((res) => {
        if (res.status >= 500) {
          return caches.match(e.request)
        } else {
          return caches.open(cacheName).then((cache) => {
            cache.put(e.request, res.clone());
            return res;
          });
        }
      })
    );
  }
});