Add "cache" source support.

docs/index.bs

@@ -1565,7 +1565,7 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
         URLPatternCompatible urlPattern;
       };
 
-      enum RouterSource { "fetch-event", "network" };
+      enum RouterSource { "cache", "fetch-event", "network" };
     </pre>
 
     <section>
@@ -3110,7 +3110,15 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
       1. If |request|'s [=request/destination=] is either "<code>embed</code>" or "<code>object</code>", then:
           1. Return null.
       1. Else if |registration|'s <a>active worker</a>'s [=service worker/list of router rules=] is [=list/is not empty=]:
-          1. If running [=Get Router Source=] algorithm with |registration|'s <a>active worker</a> and |request| returns "network", return null.
+          1. Let |source| be a result of running [=Get Router Source=] algorithm with |registration|'s <a>active worker</a> and |request|.
+          1. If |source| is "network", return null.
+          1. Else if |source| is "cache":
+              1. Let |cache promise| be a result of {{cache/matchAll(|request|, {})}}
+              1. [=Upon fulfillment=] of |cache promise| with |cache response|:
+                  1. If |cache response| is not a {{Response}} object, return null.
+                  1. Set |response| to |cache response|.
+                  1. Return |response|
+              1. Return null.
       1. Else if |request| is a <a>non-subresource request</a>, then:
           1. If |reservedClient| is not null and is an <a>environment settings object</a>, then:
               1. If |reservedClient| is not a <a>secure context</a>, return null.