Add service worker i think

index.html

@@ -141,6 +141,11 @@ <h2>
     </div>
 </footer>
     <script src="js/christmasmusic.js"></script>
+<script>
+  if (typeof navigator.serviceWorker !== 'undefined') {
+    navigator.serviceWorker.register('js/pwabuilder-sw.js')
+  }
+</script>
 </body>
 <script src="js/twemoji.js"></script>
 </html>

js/pwabuilder-sw.js

@@ -0,0 +1,54 @@
+// This is the service worker with the combined offline experience (Offline page + Offline copy of pages)
+
+const CACHE = "pwabuilder-offline-page";
+
+importScripts('https://storage.googleapis.com/workbox-cdn/releases/5.1.2/workbox-sw.js');
+
+// TODO: replace the following with the correct offline fallback page i.e.: const offlineFallbackPage = "offline.html";
+const offlineFallbackPage = "ToDo-replace-this-name.html";
+
+self.addEventListener("message", (event) => {
+  if (event.data && event.data.type === "SKIP_WAITING") {
+    self.skipWaiting();
+  }
+});
+
+self.addEventListener('install', async (event) => {
+  event.waitUntil(
+    caches.open(CACHE)
+      .then((cache) => cache.add(offlineFallbackPage))
+  );
+});
+
+if (workbox.navigationPreload.isSupported()) {
+  workbox.navigationPreload.enable();
+}
+
+workbox.routing.registerRoute(
+  new RegExp('/*'),
+  new workbox.strategies.StaleWhileRevalidate({
+    cacheName: CACHE
+  })
+);
+
+self.addEventListener('fetch', (event) => {
+  if (event.request.mode === 'navigate') {
+    event.respondWith((async () => {
+      try {
+        const preloadResp = await event.preloadResponse;
+
+        if (preloadResp) {
+          return preloadResp;
+        }
+
+        const networkResp = await fetch(event.request);
+        return networkResp;
+      } catch (error) {
+
+        const cache = await caches.open(CACHE);
+        const cachedResp = await cache.match(offlineFallbackPage);
+        return cachedResp;
+      }
+    })());
+  }
+});