Document exactly what works with the Link header

This adds "has `Link` processing" to the link types table, which currently only has "Yes" values for preload and preconnect.

Closes #4224, and removes the reference to it in the spec. That issue was about the general lack of specification for the Link header, which has been resolved. #8741 tracks potentially adding Link header support for rel=stylesheet, but there's no need to retain notices in the spec about the possibility of this nonstandard feature. (The note was also quite incomplete, as it didn't talk about things like the cascade, document.styleSheets, etc.)

Author: domenic

source

@@ -14980,12 +14980,9 @@ interface <dfn interface>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   processing options</span>. This algorithm defines whether and how they react to appearing in an
   HTTP `<code data-x="http-link">Link</code>` response header.</p>
 
-  <p class="XXX">The processing of `<code data-x="http-link">Link</code>` headers is not defined for
-  all link relation types. In particular some implementations might process types that have a no-op
-  <span>process link headers</span> algorithm, and in doing so influence a <code>Document</code>'s
-  <span>script-blocking style sheet counter</span>. See
-  <a href="https://github.com/whatwg/html/issues/4224">issue #4224</a> for discussion on
-  integrating this into the spec.</p>
+  <p class="note">For most link types, this algorithm does nothing. The <a
+  href="#table-link-relations">summary table</a> is a good reference to quickly know whether a link
+  type has defined <span>process a link header</span> steps.</p>
 
   <p>A <dfn>link processing options</dfn> is a <span>struct</span>. It has the following
   <span data-x="struct item">items</span>:</p>
@@ -17038,8 +17035,8 @@ console.log(style.disabled); // false</code></pre>
   </ul>
 
   <p class="XXX">It is expected that counterparts to the above rules also apply to
-  <code>&lt;?xml-stylesheet?></code> PIs and HTTP `<code data-x="http-link">Link</code>` headers.
-  However, this has not yet been thoroughly investigated.</p>
+  <code>&lt;?xml-stylesheet?></code> PIs. However, this has not yet been thoroughly
+  investigated.</p>
 
   <p>A <code>Document</code> has a <dfn>script-blocking style sheet counter</dfn>, which is a
   number, initially 0.</p>
@@ -24869,12 +24866,13 @@ document.body.appendChild(wbr);</code></pre>
   <p>New link types that are to be implemented by web browsers are to be added to this standard. The
   remainder can be <span data-x="concept-rel-extensions">registered as extensions</span>.</p>
 
-  <table class="yesno">
+  <table class="yesno" id="table-link-relations">
    <thead>
     <tr>
      <th rowspan="2">Link type</th>
      <th colspan="3">Effect on...</th>
      <th rowspan="2"><span>body-ok</span></th>
+     <th rowspan="2">Has `<code data-x="http-link">Link</code>` processing</th>
      <th rowspan="2">Brief description</th>
     </tr>
     <tr>
@@ -24886,18 +24884,20 @@ document.body.appendChild(wbr);</code></pre>
    <tbody>
 
     <tr>
-     <td><code data-x="rel-alternate">alternate</code></td> <!-- fourth most used rel value -->
+     <td><code data-x="rel-alternate">alternate</code></td>
      <td colspan="2"><span>Hyperlink</span></td>
      <td><em>not allowed</em></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Gives alternate representations of the current document.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-canonical">canonical</code></td> <!-- third most used rel value -->
+     <td><code data-x="rel-canonical">canonical</code></td>
      <td><span>Hyperlink</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Gives the preferred URL for the current document.</td>
     </tr>
 
@@ -24906,49 +24906,52 @@ document.body.appendChild(wbr);</code></pre>
      <td colspan="2"><span>Hyperlink</span></td>
      <td><em>not allowed</em></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Gives a link to the author of the current document or article.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-bookmark">bookmark</code></td> <!-- fourth most used <a rel> value -->
+     <td><code data-x="rel-bookmark">bookmark</code></td>
      <td><em>not allowed</em></td>
      <td><span>Hyperlink</span></td>
      <td><em>not allowed</em></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Gives the permalink for the nearest ancestor section.</td>
     </tr>
 
-    <!-- <code data-x="rel-contact">contact</code></td> is not here because it's XFN (8th most used <a rel> value) -->
-
     <tr>
      <td><code data-x="rel-dns-prefetch">dns-prefetch</code></td>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="yes"> Yes </td>
+     <td class="no"> &middot; </td>
      <td>Specifies that the user agent should preemptively perform DNS resolution for the target resource's <span>origin</span>.</td>
     </tr>
 
-    <!-- fifth and sixth most used <a rel> value (sixth is "external nofollow") -->
     <tr>
      <td><code data-x="rel-external">external</code></td>
      <td><em>not allowed</em></td>
      <td colspan="2"><span data-x="hyperlink annotation">Annotation</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Indicates that the referenced document is not part of the same site as the current document.</td>
     </tr>
 
     <tr>
      <td><code data-x="rel-help">help</code></td>
      <td colspan="3"><span>Hyperlink</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Provides a link to context-sensitive help.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-icon">icon</code></td> <!-- second most used rel value -->
+     <td><code data-x="rel-icon">icon</code></td>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Imports an icon to represent the current document.</td>
     </tr>
 
@@ -24957,6 +24960,7 @@ document.body.appendChild(wbr);</code></pre>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Imports or links to an <span>application manifest</span>. <ref spec=MANIFEST></td>
     </tr>
 
@@ -24965,30 +24969,34 @@ document.body.appendChild(wbr);</code></pre>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="yes"> Yes </td>
+     <td class="no"> &middot; </td>
      <td>Specifies that the user agent must preemptively <span data-x="fetch a single module script">fetch the module
      script</span> and store it in the document's <span data-x="concept-document-module-map">module map</span> for later
      evaluation. Optionally, the module's dependencies can be fetched as well.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-license">license</code></td> <!-- seventh most used <a rel> value -->
+     <td><code data-x="rel-license">license</code></td>
      <td colspan="3"><span>Hyperlink</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Indicates that the main content of the current document is covered by the copyright license described by the referenced document.</td>
     </tr>
 
     <tr>
      <td><code data-x="rel-next">next</code></td>
      <td colspan="3"><span>Hyperlink</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Indicates that the current document is a part of a series, and that the next document in the series is the referenced document.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-nofollow">nofollow</code></td> <!-- most used <a rel> value (and sixth most used is "external nofollow") -->
+     <td><code data-x="rel-nofollow">nofollow</code></td>
      <td><em>not allowed</em></td>
      <td colspan="2"><span data-x="hyperlink annotation">Annotation</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Indicates that the current document's original author or publisher does not endorse the referenced document.</td>
     </tr>
 
@@ -24997,6 +25005,7 @@ document.body.appendChild(wbr);</code></pre>
      <td><em>not allowed</em></td>
      <td colspan="2"><span data-x="hyperlink annotation">Annotation</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Creates a <span>top-level traversable</span> with a non-<span>auxiliary browsing
      context</span> if the hyperlink would otherwise create one that was auxiliary (i.e., has an
      appropriate <code data-x="attr-hyperlink-target">target</code> attribute value).</td>
@@ -25007,6 +25016,7 @@ document.body.appendChild(wbr);</code></pre>
      <td><em>not allowed</em></td>
      <td colspan="2"><span data-x="hyperlink annotation">Annotation</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>No `<code data-x="http-referer">Referer</code>` (sic) header will be included.
      Additionally, has the same effect as <code data-x="rel-noopener">noopener</code>.</td>
     </tr>
@@ -25016,6 +25026,7 @@ document.body.appendChild(wbr);</code></pre>
      <td><em>not allowed</em></td>
      <td colspan="2"><span data-x="hyperlink annotation">Annotation</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Creates an <span>auxiliary browsing context</span> if the hyperlink would otherwise create
      a <span>top-level traversable</span> with a non-<span>auxiliary browsing context</span> (i.e.,
      has "<code data-x="">_blank</code>" as <code data-x="attr-hyperlink-target">target</code>
@@ -25027,6 +25038,7 @@ document.body.appendChild(wbr);</code></pre>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="yes"> Yes </td>
+     <td class="no"> &middot; </td>
      <td>Gives the address of the pingback server that handles pingbacks to the current document.</td>
     </tr>
 
@@ -25035,6 +25047,7 @@ document.body.appendChild(wbr);</code></pre>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="yes"> Yes </td>
+     <td class="yes"> Yes </td>
      <td>Specifies that the user agent should preemptively connect to the target resource's <span>origin</span>.</td>
     </tr>
 
@@ -25043,6 +25056,7 @@ document.body.appendChild(wbr);</code></pre>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="yes"> Yes </td>
+     <td class="no"> &middot; </td>
      <td>Specifies that the user agent should preemptively <span data-x="concept-fetch">fetch</span> and cache the target resource as it is likely to be required for a followup <span data-x="navigate">navigation</span>.</td>
     </tr>
 
@@ -25051,6 +25065,7 @@ document.body.appendChild(wbr);</code></pre>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="yes"> Yes </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-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>
 
@@ -25059,40 +25074,44 @@ document.body.appendChild(wbr);</code></pre>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="yes"> Yes </td>
+     <td class="no"> &middot; </td>
      <td>Specifies that the user agent should preemptively <span data-x="concept-fetch">fetch</span> the target resource and process it in a way that helps deliver a faster response in the future.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-prev">prev</code></td> <!-- prev is used more than previous -->
+     <td><code data-x="rel-prev">prev</code></td>
      <td colspan="3"><span>Hyperlink</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Indicates that the current document is a part of a series, and that the previous document in the series is the referenced document.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-search">search</code></td> <!-- used quite a bit -->
+     <td><code data-x="rel-search">search</code></td>
      <td colspan="3"><span>Hyperlink</span></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Gives a link to a resource that can be used to search through the current document and its related pages.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-stylesheet">stylesheet</code></td> <!-- most commonly used <link rel> value, variants came in 7th, 8th, 12th, 17th... -->
+     <td><code data-x="rel-stylesheet">stylesheet</code></td>
      <td><span data-x="external resource link">External Resource</span></td>
      <td colspan="2"><em>not allowed</em></td>
      <td class="yes"> Yes </td>
+     <td class="no"> &middot; </td>
      <td>Imports a style sheet.</td>
     </tr>
 
     <tr>
-     <td><code data-x="rel-tag">tag</code></td> <!-- second and third most used <a rel> value (third is technically "category tag"). -->
+     <td><code data-x="rel-tag">tag</code></td>
      <td><em>not allowed</em></td>
      <td><span>Hyperlink</span></td>
      <td><em>not allowed</em></td>
      <td class="no"> &middot; </td>
+     <td class="no"> &middot; </td>
      <td>Gives a tag (identified by the given address) that applies to the current document.</td>
     </tr>
-
    </tbody>
   </table>