Define link processing for prefetch

Prefetch is simply a fetch with `prefetch-src` CSP and no
post-processing of the resource.

Closes whatwg#5229

source

@@ -26048,15 +26048,45 @@ 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>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>Set <var>request</var>'s <span data-x="concept-request-destination">destination</span>
+   to "<code data-x="">prefetch</code>".</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>Set <var>request</var>'s <span
+   data-x="concept-request-header-list">header list</span> to « (`<code
+   data-x="">Sec-Purpose</code>`, `<code data-x="">Prefetch</code>) ».</p></li>
+
+   <li><p>The user agent should <span data-x="concept-fetch">fetch</span> <var>request</var>. User
+   agents may delay the fetching of <var>request</var> to prioritize othere 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>