Specify <link rel=prefetch>

Prefetch is simply a fetch, which populates the HTTP cache, with no post-processing of the resource and with a special header Sec-Purpose: prefetch. (The latter is specified in whatwg/fetch#1576.)

Closes #5229.
Closes w3c/resource-hints#86.
Closes w3c/resource-hints#74.
Closes whatwg/fetch#1008.

source

@@ -26048,15 +26048,58 @@ document.body.appendChild(wbr);</code></pre>
   link</span>. This keyword is <span>body-ok</span>.</p>
 
   <p>The <code data-x="rel-prefetch">prefetch</code> keyword indicates that preemptively <span
-  data-x="concept-fetch">fetching</span> and caching the specified resource is likely to be
-  beneficial, as it is highly likely that the user will require this resource for future
-  navigations. <span w-nodev>User agents must implement the processing model of the <code
-  data-x="rel-prefetch">prefetch</code> keyword described in <cite>Resource Hints</cite>.</span>
-  <ref spec=RESOURCEHINTS></p>
+  data-x="concept-fetch">fetching</span> and caching the specified resource or same-site document is
+  likely to be beneficial, as it is highly likely that the user will require this resource for
+  future navigations.</p>
 
   <p>There is no default type for resources given by the <code data-x="rel-prefetch">prefetch</code>
   keyword.</p>
 
+  <p>The <span>fetch and process the linked resource</span> algorithm for <code
+  data-x="rel-prefetch">prefetch</code> links, given a <code>link</code> element
+  <var>el</var>, is as follows:</p>
+
+  <ol>
+   <li><p>If <var>el</var>'s <code data-x="attr-link-href">href</code> attribute's value is the
+   empty string, then return.</p></li>
+
+   <li><p>Let <var>options</var> be the result of <span data-x="create link options from
+   element">creating link options</span> from <var>el</var>.</p></li>
+
+   <li><p>Set <var>options</var>'s <span data-x="link options destination">destination</span> to
+   the empty string.</p></li>
+
+   <li><p>Let <var>request</var> be the result of <span data-x="create a link request">creating a
+   link request</span> given <var>options</var>.</p></li>
+
+   <li><p>If <var>request</var> is null, then return.</p></li>
+
+   <li><p>Set <var>request</var>'s <span data-x="concept-request-initiator">initiator</span> to
+   "<code data-x="">prefetch</code>".</p></li>
+
+   <li>
+    <p>Let <var>processPrefetchResponse</var> be the following steps given a <span
+    data-x="concept-response">response</span> <var>response</var> and null, failure, or a
+    <span>byte sequence</span> <var>bytesOrNull</var>:</p>
+
+    <ol>
+     <li><p>If <var>response</var> is a <span>network error</span>, <span
+     data-x="concept-event-fire">fire an event</span> named <code
+     data-x="event-error">error</code> at <var>el</var>.</p></li>
+
+     <li><p>Otherwise, <span data-x="concept-event-fire">fire an event</span> named <code
+     data-x="event-load">load</code> at <var>el</var>.</p></li>
+    </ol>
+   </li>
+
+   <li><p>The user agent should <span data-x="concept-fetch">fetch</span> <var>request</var>, with
+   <i data-x="processResponseConsumeBody">processResponseConsumeBody</i> set to
+   <var>processPrefetchResponse</var>. User agents may delay the fetching of <var>request</var> to
+   prioritize other requests that are necessary for the current document.</p></li>
+  </ol>
+
+  <p>The <span>process a link header</span> steps for this type of linked resource are to do
+  nothing.</p>
 
   <h5>Link type "<dfn for="link/rel" attr-value><code
   data-x="rel-preload">preload</code></dfn>"</h5>