Reintroduce MediaMetadata artwork (a URL)

mediasession.bs

@@ -88,6 +88,7 @@ urlPrefix: https://html.spec.whatwg.org/multipage/; spec: HTML
 urlPrefix: https://url.spec.whatwg.org/;
     type: dfn; urlPrefix: #concept-
         text: url parser
+        text: url serializer
         text: event listener
 urlPrefix: https://fetch.spec.whatwg.org/; spec: FETCH
     type: dfn; urlPrefix: #concept-
@@ -289,8 +290,18 @@ attribute must return the <a>media session</a>'s <a>kind</a>.
 
 The <dfn attribute for="MediaSession"><code>metadata</code></dfn> attribute, on
 getting, must return the <a>media session</a>'s <a lt="media session
-metadata">metadata</a>. On setting, the <a>media session</a>'s <a lt="media
-session metadata">metadata</a> must be set to the new value.
+metadata">metadata</a>. On setting, user agent must run the following steps:
+
+<ol>
+  <li>
+    Set the <a>media session</a>'s <a lt="media session metadata">metadata</a>
+    to the new value.
+  </li>
+  <li>
+    Run the <a>fetch steps</a> for <a>media session</a>'s <a lt="media session
+    metadata">metadata</a>.
+  </li>
+</ol>
 
 The <dfn method for="MediaSession"><code>activate()</code></dfn> method, when
 invoked, must run these steps:
@@ -1123,15 +1134,23 @@ interface MediaMetadata {
   readonly attribute DOMString title;
   readonly attribute DOMString artist;
   readonly attribute DOMString album;
+  readonly attribute USVString artwork;
 };
 
 dictionary MediaMetadataInit {
   DOMString title = "";
   DOMString artist = "";
   DOMString album = "";
+  USVString artwork;
 };
 </pre>
 
+A {{MediaMetadata}} object has a <dfn for="MediaMetadata">title</dfn>, an <dfn
+for="MediaMetadata">artist</dfn>, and an <dfn for="MediaMetadata">album</dfn>.
+
+A {{MediaMetadata}} object <em>can</em> have an <dfn for="MediaMetadata">artwork
+URL</dfn> and a <dfn for="MediaMetadata">artwork resource</dfn>.
+
 The <dfn constructor
 for="MediaMetadata"><code>MediaMetadata(<var>init</var>)</code></dfn>
 constructor, when invoked, must run the following steps:
@@ -1141,16 +1160,79 @@ constructor, when invoked, must run the following steps:
     Let <var>metadata</var> be a new {{MediaMetadata}} object.
   </li>
   <li>
-    Set <var>metadata</var>'s {{MediaMetadata/title}}, {{MediaMetadata/artist}},
-    and {{MediaMetadata/album}} attributes to the values of <var>init</var>'s
-    {{MediaMetadataInit/title}}, {{MediaMetadataInit/artist}}, and
-    {{MediaMetadataInit/album}} members, respectively.
+    Set <var>metadata</var>'s <a>title</a> to <var>init</var>'s
+    <code>title</code>.
+  </li>
+  <li>
+    Set <var>metadata</var>'s <a>artist</a> to <var>init</var>'s
+    <code>artist</code>.
+  </li>
+  <li>
+    Set <var>metadata</var>'s <a>album</a> to <var>init</var>'s
+    <code>album</code>.
+  </li>
+  <li>
+    Let <var>baseURL</var> be the <a>API base URL</a> specified by the <a>entry
+    settings object</a>.
+  </li>
+  <li>
+    If <var>init</var>'s <code>artwork</code> is present,
+    <a lt="url parser">parse</a> it using <var>baseURL</var>, and if that does
+    not return failure, set <var>metadata</var>'s <a>artwork URL</a> to the
+    return value. (Otherwise <a>artwork URL</a> is not set.)
   </li>
   <li>
     Return <var>metadata</var>.
   </li>
 </ol>
 
+The <dfn attribute for="MediaMetadata"><code>title</code></dfn> attribute must
+return the {{MediaMetadata}} objects's <a>title</a>.
+
+The <dfn attribute for="MediaMetadata"><code>artist</code></dfn> attribute must
+return the {{MediaMetadata}} objects's <a>artist</a>.
+
+The <dfn attribute for="MediaMetadata"><code>album</code></dfn> attribute must
+return the {{MediaMetadata}} objects's <a>album</a>.
+
+The <dfn attribute for="MediaMetadata"><code>artwork</code></dfn> attribute must
+return the {{MediaMetadata}} objects's <a>artwork URL</a>, <a lt="url
+serializer">serialized</a>, and the empty string if there is no <a>artwork
+URL</a> otherwise.
+
+The <dfn>fetch steps</dfn> for a given {{MediaMetadata}} object
+<var>metadata</var> are:
+
+<ol>
+  <!-- XXX https://www.w3.org/Bugs/Public/show_bug.cgi?id=24055 -->
+  <li>
+    If <var>metadata</var> has no <a>artwork URL</a>, then terminate these
+    steps.
+  </li>
+  <li>
+    If the platform supports displaying media artwork, <a>fetch</a>
+    <var>metadata</var>'s <a>artwork URL</a>.
+
+    Then, <a>in parallel</a>:
+
+    <ol>
+      <li>
+        Wait for the <a>response</a>.
+      </li>
+      <li>
+        If the <a>response</a>'s <a>internal response</a>'s
+        <a lt="response type">type</a> is <i>default</i>, attempt to decode the
+        resource as image.
+      </li>
+      <li>
+        If the image format is supported, set <var>metadata</var>'s <a>artwork
+        resource</a> to the decoded resource. (Otherwise <var>metadata</var> has
+        no <a>artwork resource</a>.)
+      </li>
+    </ol>
+  </li>
+</ol>
+
 <h2 id="extensions-to-the-htmlmediaelement-interface">Extensions to the
 {{HTMLMediaElement}} interface</h2>
 
@@ -1353,6 +1435,7 @@ When the user agent is to <dfn>resume a web audio object</dfn> for a given
       title: "Episode Title",
       artist: "Podcast Host",
       album: "Podcast Title",
+      artwork: "podcast.jpg",
     });
   </pre>
 </div>
@@ -1385,6 +1468,7 @@ When the user agent is to <dfn>resume a web audio object</dfn> for a given
         title: event.target == audio1 ? "Chapter 1" : "Chapter 2",
         artist: "An Author",
         album: "A Book",
+        artwork: "cover.jpg",
       });
     }