Update <link>'s processing model to work well for preload

This allows the preload spec to delegate to HTML's "obtain the resource"
algorithm for elements, instead of creating a copy of it. The
latter approach has been attempted, but got out of date very quickly;
see w3c/preload#91 for more details.

This also updates rel="preload" to account for recent changes to Fetch
that introduced the concept of a "potential destination" for use here.
See whatwg/fetch#547.

Notably, part of this integration is ignoring invalid as="" attributes,
which now includes empty ones. No error event is fired.

Finally, it also allows other specifications to override the delaying of
the load event, as the preload spec does.

source

@@ -2699,6 +2699,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
        <li><dfn data-x="concept-request-initiator" data-x-href="https://fetch.spec.whatwg.org/#concept-request-initiator">initiator</dfn></li>
        <li><dfn data-x="concept-request-type" data-x-href="https://fetch.spec.whatwg.org/#concept-request-type">type</dfn></li>
        <li><dfn data-x="concept-request-destination" data-x-href="https://fetch.spec.whatwg.org/#concept-request-destination">destination</dfn></li>
+       <li><dfn data-x="concept-potential-destination" data-x-href="https://fetch.spec.whatwg.org/#concept-potential-destination">potential destination</dfn></li>
+       <li><dfn data-x="concept-potential-destination-translate" data-x-href="https://fetch.spec.whatwg.org/#concept-potential-destination-translate">translating</dfn> a <span data-x="concept-potential-destination">potential destination</span></li>
        <li><dfn data-x="concept-request-priority" data-x-href="https://fetch.spec.whatwg.org/#concept-request-priority">priority</dfn></li>
        <li><dfn data-dfn-for="request" data-x="concept-request-origin" data-x-href="https://fetch.spec.whatwg.org/#concept-request-origin">origin</dfn></li>
        <li><dfn data-x="concept-request-referrer" data-x-href="https://fetch.spec.whatwg.org/#concept-request-referrer">referrer</dfn></li>
@@ -13110,17 +13112,21 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   <hr>
 
   <p>The <dfn><code data-x="attr-link-as">as</code></dfn> attribute specifies the <span
-  data-x="concept-request-destination">destination</span> for a preload request for the resource
-  given by the <code data-x="attr-link-href">href</code> attribute. It is an <span>enumerated
-  attribute</span>. Each <span data-x="concept-request-destination">request destination</span>,
-  including the empty string, is a keyword for this attribute, mapping to a state of the same name.
-  The attribute's <i data-x="missing value default">missing value default</i> and <i data-x="invalid
-  value default">invalid value default</i> are both the empty string state. The attribute may be
-  omitted. The attribute must not be specified on <code>link</code> elements that do not have a
-  <code data-x="attr-link-rel">rel</code> attribute that contains the
-  <code data-x="rel-preload">preload</code> keyword.<span w-nodev> User agents must implement the
-  processing model of the <code data-x="attr-link-as">as</code> attribute described in
-  <cite>Preload</cite>. <ref spec=FETCH> <ref spec=PRELOAD></span></p>
+  data-x="concept-potential-destination">potential destination</span> for a preload request for the
+  resource given by the <code data-x="attr-link-href">href</code> attribute. It is an
+  <span>enumerated attribute</span>. Each <span data-x="concept-potential-destination">potential
+  destination</span> is a keyword for this attribute, mapping to a state of the same name. The
+  attribute must be specified on <code>link</code> elements that have a <code
+  data-x="attr-link-rel">rel</code> attribute that contains the <code
+  data-x="rel-preload">preload</code> keyword, but must not be specified on <code>link</code>
+  elements which do not. <span w-nodev>The processing model for how the <code
+  data-x="attr-link-as">as</code> attribute is used is given in the <span
+  data-x="concept-link-obtain">steps to obtain the resource</span>.</span></p>
+
+  <p class="note">The attribute does not have a <i data-x="missing value default">missing value
+  default</i> or <i data-x="invalid value default">invalid value default</i>, meaning that invalid
+  or missing values for the attribute map to no state. This is accounted for in the processing
+  model.</p>
 
   <hr>
 
@@ -13327,6 +13333,22 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
    <code>link</code> element's <code data-x="attr-link-referrerpolicy">referrerpolicy</code>
    attribute.</p></li>
 
+   <li>
+    <p>If the <code data-x="attr-link-rel">rel</code> attribute contains the <code
+    data-x="rel-preload">preload</code> keyword, then:</p>
+
+    <ol>
+     <li><p>Let <var>as</var> be the current state of the <code data-x="attr-link-as">as</code>
+     attribute.</p></li>
+
+     <li><p>If <var>as</var> is no state, then return.</p></li>
+
+     <li><p>Set <var>request</var>'s <span data-x="concept-request-destination">destination</span>
+     to the result of <span data-x="concept-potential-destination-translate">translating</span>
+     <var>as</var>.</p></li>
+    </ol>
+   </li>
+
    <!--FETCH--><li><p><span data-x="concept-fetch">Fetch</span> <var>request</var>.</p></li>
 
   </ol>
@@ -13353,10 +13375,11 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   <p>The <span>task source</span> for these <span data-x="concept-task">tasks</span> is the
   <span>DOM manipulation task source</span>.</p>
 
-  <p>The element must <span>delay the load event</span> of the element's <span>node document</span>
-  until all the attempts to obtain the resource and its <span>critical subresources</span> are
-  complete.  (Resources that the user agent has not yet attempted to obtain, e.g. because it is
-  waiting for the resource to be needed, do not <span>delay the load event</span>.)</p>
+  <p>Unless otherwise specified for a given <code data-x="attr-link-rel">rel</code> keyword, the
+  element must <span>delay the load event</span> of the element's <span>node document</span> until
+  all the attempts to obtain the resource and its <span>critical subresources</span> are complete.
+  (Resources that the user agent has not yet attempted to obtain, e.g. because it is waiting for the
+  resource to be needed, do not <span>delay the load event</span>.)</p>
 
   <h5>Processing `<code data-x="http-link">Link</code>` headers</h5>
 
@@ -23357,7 +23380,7 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
      <td><span data-x="external resource link">External Resource</span></td>
      <td><em>not allowed</em></td>
      <td class="yes"> Yes </td>
-     <td>Specifies that the user agent must preemptively <span data-x="concept-fetch">fetch</span> and cache the target resource for current <span data-x="navigate">navigation</span> according to the <span data-x="concept-request-destination">destination</span> given by the <code data-x="attr-link-as">as</code> attribute (and the <span data-x="concept-request-priority">priority</span> associated with that <span data-x="concept-request-destination">destination</span>).</td>
+     <td>Specifies that the user agent must preemptively <span data-x="concept-fetch">fetch</span> and cache the target resource for current <span data-x="navigate">navigation</span> according to the <span data-x="concept-potential-destination">potential destination</span> given by the <code data-x="attr-link-as">as</code> attribute (and the <span data-x="concept-request-priority">priority</span> associated with the <span data-x="concept-potential-destination-translate">corresponding</span> <span data-x="concept-request-destination">destination</span>).</td>
     </tr>
 
     <tr>
@@ -23996,13 +24019,15 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
 
   <p>The <code data-x="rel-preload">preload</code> keyword indicates that the user agent must
   preemptively <span data-x="concept-fetch">fetch</span> and cache the specified resource according
-  to the <span data-x="concept-request-destination">destination</span> given by the <code
-  data-x="attr-link-as">as</code> attribute (and the <span
-  data-x="concept-request-priority">priority</span> associated with that <span
+  to the <span data-x="concept-potential-destination">potential destination</span> given by the
+  <code data-x="attr-link-as">as</code> attribute (and the <span
+  data-x="concept-request-priority">priority</span> associated with the <span
+  data-x="concept-potential-destination-translate">corresponding</span> <span
   data-x="concept-request-destination">destination</span>), as it is highly likely that the user
   will require this resource for the current navigation. <span w-nodev>User agents must implement
   the processing model of the <code data-x="rel-preload">preload</code> keyword described in
-  <cite>Preload</cite>.</span> <ref spec=PRELOAD></p>
+  <cite>Preload</cite>, as well as in this specification's <span data-x="concept-link-obtain">obtain
+  the resource</span> algorithm.</span> <ref spec=PRELOAD></p>
 
   <p>There is no default type for resources given by the <code data-x="rel-preload">preload</code>
   keyword.</p>
@@ -117601,8 +117626,8 @@ interface <dfn>External</dfn> {
     <tr>
      <th> <code data-x="">as</code>
      <td> <code data-x="attr-link-as">link</code>
-     <td> Destination for a preload request (for <code data-x="attr-link-rel">rel</code>="<code data-x="rel-preload">preload</code>")
-     <td> <span data-x="concept-request-destination">Destination keyword</span>
+     <td> <span data-x="concept-potential-destination">Potential destination</span> for a preload request (for <code data-x="attr-link-rel">rel</code>="<code data-x="rel-preload">preload</code>")
+     <td> <span data-x="concept-potential-destination">Potential destination</span>
     <tr>
      <th> <code data-x="">async</code>
      <td> <code data-x="attr-script-async">script</code>