Make <link rel=canonical> a standard rel keyword

This change adds “canonical” to the HTML spec as a standard keyword
allowed in the value of the rel="" attribute for the <link> element.

Fixes whatwg#2351.

source

@@ -12880,6 +12880,8 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   used are allowed according to the definitions in this specification, then the element does not
   create any links.</span></p>
 
+  <div w-nodev>
+
   <p><code data-x="attr-link-rel">rel</code>'s
   <span data-x="concept-supported-tokens">supported tokens</span> are the keywords defined in
   <a href="#linkTypes">HTML link types</a> which are allowed on <code>link</code> elements, impact
@@ -12901,6 +12903,15 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   tokens</span> must only include the tokens from this list that the user agent implements the
   processing model for.</p>
 
+  <p class="note">Theoretically a user agent could support the processing model for the <code
+  data-x="rel-canonical">canonical</code> keyword — if it were a search engine that executed
+  JavaScript. But in practice that's quite unlikely. So in most cases, <code
+  data-x="rel-canonical">canonical</code> ought not be included in <code
+  data-x="attr-link-rel">rel</code>'s <span data-x="concept-supported-tokens">supported
+  tokens</span>.</p>
+
+  </div>
+
   <p>A <code>link</code> element must have either a <code data-x="attr-link-rel">rel</code>
   attribute or an <code data-x="attr-itemprop">itemprop</code> attribute, but not both.</p>
 
@@ -23168,13 +23179,21 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
    <tbody>
 
     <tr>
-     <td><code data-x="rel-alternate">alternate</code></td> <!-- second most used <link rel> value -->
+     <td><code data-x="rel-alternate">alternate</code></td> <!-- fourth most used rel value -->
      <td><span>Hyperlink</span></td>
      <td><span>Hyperlink</span></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><span>Hyperlink</span></td>
+     <td><em>not allowed</em></td>
+     <td class="no"> &middot; </td>
+     <td>Gives the preferred URL for the current document.</td>
+    </tr>
+
     <tr>
      <td><code data-x="rel-author">author</code></td>
      <td><span>Hyperlink</span></td>
@@ -23219,7 +23238,7 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
     </tr>
 
     <tr>
-     <td><code data-x="rel-icon">icon</code></td> <!-- link rel="shortcut icon" and its ilk are the fourth, sixth, and ninth most used values -->
+     <td><code data-x="rel-icon">icon</code></td> <!-- second most used rel value -->
      <td><span data-x="external resource link">External Resource</span></td>
      <td><em>not allowed</em></td>
      <td class="no"> &middot; </td>
@@ -23555,6 +23574,17 @@ interface <dfn>HTMLHyperlinkElementUtils</dfn> {
   </div>
 
 
+  <h5>Link type "<dfn><code data-x="rel-canonical">canonical</code></dfn>"</h5>
+
+  <p>The <code data-x="rel-canonical">canonical</code> keyword may be used with <code>link</code>
+  element. This keyword creates a <span>hyperlink</span>.</p>
+
+  <p>The <code data-x="rel-canonical">canonical</code> keyword indicates that URL given by the <code
+  data-x="attr-link-href">href</code> attribute is the preferred URL for the current document. That
+  helps search engines reduce duplicate content, as described in more detail in <cite>The Canonical
+  Link Relation</cite> specification. <ref spec=RFC6596></p>
+
+
   <h5>Link type "<dfn><code data-x="rel-dns-prefetch">dns-prefetch</code></dfn>"</h5>
 
   <p>The <code data-x="rel-dns-prefetch">dns-prefetch</code> keyword may be used with
@@ -119630,6 +119660,9 @@ INSERT INTERFACES HERE
    <dt id="refsRFC6350">[RFC6350]</dt>
    <dd><cite><a href="https://tools.ietf.org/html/rfc6350">vCard Format Specification</a></cite>, S. Perreault. IETF.</dd>
 
+   <dt id="refsRFC6596">[RFC6596]</dt>
+   <dd><cite><a href="https://tools.ietf.org/html/rfc6596">The Canonical Link Relation</a></cite>, M. Ohye, J. Kupke. IETF.</dd>
+
    <dt id="refsRFC7303">[RFC7303]</dt>
    <dd><cite><a href="https://tools.ietf.org/html/rfc7303">XML Media Types</a></cite>, H. Thompson, C. Lilley. IETF.</dd>