Fixed references with links to archived HTML5 standard (#41)

* Replaced outdated links with Wayback Machine

Some of the documentation links are no longer working, because the AppCache has also been removed from the HTML standard. (The 404 error page explains that "Links to the multipage version of the specification are unfortunately likely to break over time.")

[Other source files of this project](https://github.com/GoogleChromeLabs/sw-appcache-behavior/blob/main/packages/appcache-polyfill-sw/index.ts#L167) link to a W3C snapshot of the living specification. However, that snapshot (which appears to be the most recent one at the W3C) seems to be too old to contain the specific content referenced from here. Hence, I propose to link to the last existing copy in the Internet Archive (which seems to match the comments in this file).

* Consistent reference with our source file

Updated the reference to be consistent throughout the code, see commit c700922#diff-40410b51384db4d3ef6d50cc67b8944a838633468afcb9177f6b7a52d8e0d429

packages/appcache-polyfill-sw/index.ts

@@ -164,7 +164,7 @@ async function noManifestBehavior(event: FetchEvent) {
   // there are multiple matches, the longest prefix wins. If there are
   // multiple prefixes of the same length in different manifest, then
   // the one we access last wins. (This might not match browser behavior.)
-  // See https://www.w3.org/TR/2011/WD-html5-20110525/offline.html#concept-appcache-matches-fallback
+  // See https://web.archive.org/web/20201129180031/https://html.spec.whatwg.org/multipage/offline.html#concept-appcache-matches-fallback
   const manifestURLToHashes: ManifestURLToHashes =
     await storage.get('ManifestURLToHashes');
 

packages/appcache-polyfill-window/index.ts

@@ -60,7 +60,7 @@ async function addToCache(hash: string, urls: Array<string>) {
   const cache = await caches.open(hash);
 
   await Promise.all(urls.map(async (url) => {
-    // See Item 18.3 of https://html.spec.whatwg.org/multipage/browsers.html#downloading-or-updating-an-application-cache
+    // See Item 18.3 of https://web.archive.org/web/20201129180031/https://html.spec.whatwg.org/multipage/offline.html#downloading-or-updating-an-application-cache
     const init: RequestInit = {
       credentials: 'include',
       headers: [['X-Use-Fetch', 'true']],
@@ -83,7 +83,7 @@ async function addToCache(hash: string, urls: Array<string>) {
         return;
       }
 
-      // See Item 18.5 of https://html.spec.whatwg.org/multipage/browsers.html#downloading-or-updating-an-application-cache
+      // See Item 18.5 of https://web.archive.org/web/20201129180031/https://html.spec.whatwg.org/multipage/offline.html#downloading-or-updating-an-application-cache
       if (response.status !== 404 && response.status !== 410) {
         // Assuming this isn't a 200, 404 or 410, we want the catch to
         // trigger, which will cause any previously cached Response for this
@@ -104,15 +104,15 @@ async function addToCache(hash: string, urls: Array<string>) {
 }
 
 async function checkManifestVersion(manifestUrl: string) {
-  // See Item 4 of https://html.spec.whatwg.org/multipage/browsers.html#downloading-or-updating-an-application-cache
+  // See Item 4 of https://web.archive.org/web/20201129180031/https://html.spec.whatwg.org/multipage/offline.html#downloading-or-updating-an-application-cache
   const init: RequestInit = {
     credentials: 'include',
     headers: [['X-Use-Fetch', 'true']],
   };
   const manifestRequest = new Request(manifestUrl, init);
 
   let manifestResponse = await fetch(manifestRequest);
-  // See Item 6 of https://html.spec.whatwg.org/multipage/offline.html#downloading-or-updating-an-application-cache
+  // See Item 6 of https://web.archive.org/web/20201129180031/https://html.spec.whatwg.org/multipage/offline.html#downloading-or-updating-an-application-cache
   if (manifestResponse.status === 404 || manifestResponse.status === 410) {
     // TODO: Double check that this is a reasonable approach.
     // If the manifest has explicitly been removed, then remove our saved state
@@ -122,7 +122,7 @@ async function checkManifestVersion(manifestUrl: string) {
     return null;
   }
 
-  // See https://html.spec.whatwg.org/multipage/offline.html#cache-failure-steps
+  // See https://web.archive.org/web/20201129180031/https://html.spec.whatwg.org/multipage/offline.html#cache-failure-steps
   if (manifestResponse.status !== 200) {
     // TODO: Double check that this is a reasonable approach.
     // For non-200/404/410 responses, just abort.