Define the processing model for dns-prefetch

Note that this still doesn't include the CSP bits, which are also not implemented anywhere for this & preconnect. That's tracked by #9035.

Closes #9015.

source

@@ -2577,6 +2577,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#determine-the-network-partition-key">determine the network partition key</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#extract-full-timing-info">extract full timing info</dfn></li>
      <li><dfn data-x-href="https://fetch.spec.whatwg.org/#byte-sequence-as-a-body">as a body</dfn></li>
+     <li><dfn data-x-href="https://fetch.spec.whatwg.org/#resolve-an-origin">resolve an origin</dfn></li>
      <li>
       <dfn data-x="concept-response"
       data-x-href="https://fetch.spec.whatwg.org/#concept-response">response</dfn> and its
@@ -25763,9 +25764,43 @@ document.body.appendChild(wbr);</code></pre>
   <p>There is no default type for resources given by the <code
   data-x="rel-dns-prefetch">dns-prefetch</code> keyword.</p>
 
-  <p class="XXX">The processing model for <code data-x="rel-dns-prefetch">dns-prefetch</code> needs
-  to be properly specified. Tracked in <a
-  href="https://github.com/whatwg/html/issues/9015">issue #9015</a>.</p>
+  <p>The appropriate times to <span data-x="fetch and process the linked resource">fetch and
+  process</span> this type of link are:</p>
+
+  <ul>
+   <li><p>When the <span>external resource link</span> is created on a <code>link</code> element
+   that is already <span>browsing-context connected</span>.</p></li>
+   <!-- e.g. rel="" changed, href="" set... -->
+
+   <li><p>When the <span>external resource link</span>'s <code>link</code> element <span>becomes
+   browsing-context connected</span>.</p></li>
+
+   <li><p>When the <code data-x="attr-link-href">href</code> attribute of the <code>link</code>
+   element of an <span>external resource link</span> that is already <span>browsing-context
+   connected</span> is changed.</p></li>
+  </ul>
+
+  <p>The <span>fetch and process the linked resource</span> steps for this type of linked resource,
+  given a <code>link</code> element <var>el</var>, are:
+
+  <ol>
+   <li><p><span data-x="parse a URL">Parse</span> <var>el</var>'s
+   <code data-x="attr-link-href">href</code> attribute relative to <var>el</var>'s
+   <span>node document</span> and let <var>url</var> be the <span>resulting URL record</span>. If
+   this fails, return.</p></li>
+
+   <li><p>Let <var>partitionKey</var> be the result of <span data-x="determine the network
+   partition key">determining the network partition key</span> given <var>el</var>'s
+   <span>node document</span>'s <span>relevant settings object</span>.</p></li>
+
+   <li>
+    <p>The user agent should <span>resolve an origin</span> given <var>partitionKey</var> and
+    <var>url</var>'s <span data-x="concept-url-origin">origin</span>.</p>
+
+    <p class="note">As the results of this algorithm can be cached, future fetches could be
+    faster.</p>
+   </li>
+  </ol>
 
 
   <h5>Link type "<dfn attr-value for="a/rel,area/rel,form/rel"><code