Meta: add a service worker to the spec

This allows the spec to be viewed offline.

deploy.sh

@@ -75,6 +75,7 @@ else
          > $WEB_ROOT/index.intermediate.html
     node_modules/.bin/emu-algify --throwing-indicators < $WEB_ROOT/index.intermediate.html > $WEB_ROOT/index.html
     rm $WEB_ROOT/index.intermediate.html
+    cp service-worker.js $WEB_ROOT/service-worker.js
     echo "Living standard output to $WEB_ROOT"
 fi
 

index.bs

@@ -4314,3 +4314,10 @@ This standard is written by <a href="https://domenic.me/">Domenic Denicola</a>
 <a href="mailto:[email protected]">[email protected]</a>).
 
 Per <a href="https://creativecommons.org/publicdomain/zero/1.0/">CC0</a>, to the extent possible under law, the editor has waived all copyright and related or neighboring rights to this work.
+
+<script>
+"use strict";
+if ("serviceWorker" in navigator) {
+  navigator.serviceWorker.register("service-worker.js");
+}
+</script>

service-worker.js

@@ -0,0 +1,47 @@
+"use strict";
+// Largely based on https://css-tricks.com/serviceworker-for-offline/
+
+const cacheKey = "v1";
+const toCache = [
+  "/",
+  "https://resources.whatwg.org/standard.css",
+  "https://resources.whatwg.org/bikeshed.css",
+  "https://resources.whatwg.org/file-issue.js",
+  "https://resources.whatwg.org/commit-snapshot-shortcut-key.js",
+  "https://resources.whatwg.org/logo-streams.svg"
+];
+
+self.oninstall = e => {
+  e.waitUntil(caches.open(cacheKey).then(cache => cache.addAll(toCache)));
+};
+
+self.onfetch = e => {
+  if (e.request.method !== "GET") {
+    return;
+  }
+
+  e.respondWith(
+    caches.match(e.request).then(cachedResponse => {
+      // Respond with the cached response if it exists, but still do the network fetch in order to refresh the cache.
+      // Ignore network fetch errors; they just mean we won't be able to cache.
+      const networkFetchPromise = fetch(e.request).then(refreshCacheFromNetworkResponse).catch(() => {});
+
+      return cachedResponse || networkFetchPromise;
+    })
+  );
+
+  function refreshCacheFromNetworkResponse(response) {
+    const responseForCache = response.clone();
+
+    // Ignore any errors while caching.
+    caches.open(cacheKey).then(cache => cache.put(e.request, responseForCache));
+
+    return response;
+  }
+};
+
+self.onactivate = e => {
+  e.waitUntil(caches.keys().then(keys => {
+    return Promise.all(keys.filter(key => key !== cacheKey).map(key => caches.delete(key)));
+  }));
+};