rel=""'s values are unordered and unique

Last usage of duplication was removed in 16485bf and isn't supported by the API. The order is also not significant ("stylesheet alternate" is fine).

This codifies this in the semantics.

source

@@ -13312,8 +13312,8 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
 
   <p>The types of link indicated (the relationships) are given by the value of the <dfn><code
   data-x="attr-link-rel">rel</code></dfn> attribute, which, if present, must have a value that is a
-  <span>set of space-separated tokens</span>. The <a href="#linkTypes">allowed keywords and their
-  meanings</a> are defined in a later section. <span w-nodev>If the <code
+  <span>unordered set of unique space-separated tokens</span>. The <a href="#linkTypes">allowed
+  keywords and their meanings</a> are defined in a later section. <span w-nodev>If the <code
   data-x="attr-link-rel">rel</code> attribute is absent, has no keywords, or if none of the keywords
   used are allowed according to the definitions in this specification, then the element does not
   create any links.</span></p>
@@ -22523,8 +22523,8 @@ document.body.appendChild(wbr);</code></pre>
   <p><span w-dev subdfn data-x="dom-a-rellist"></span>The <dfn><code
   data-x="attr-hyperlink-rel">rel</code></dfn> attribute on <code>a</code> and <code>area</code>
   elements controls what kinds of links the elements create. The attribute's value must be a
-  <span>set of space-separated tokens</span>. The <a href="#linkTypes">allowed keywords and their
-  meanings</a> are defined below.</p>
+  <span>unordered set of unique space-separated tokens</span>. The <a href="#linkTypes">allowed
+  keywords and their meanings</a> are defined below.</p>
 
   <p><code data-x="attr-hyperlink-rel">rel</code>'s
   <span data-x="concept-supported-tokens">supported tokens</span> are the keywords defined in
@@ -119375,12 +119375,12 @@ interface <dfn>External</dfn> {
      <td> <code data-x="attr-hyperlink-rel">a</code>;
           <code data-x="attr-hyperlink-rel">area</code>
      <td> Relationship between the location in the document containing the <span>hyperlink</span> and the destination resource
-     <td> <span>Set of space-separated tokens</span>*
+     <td> <span>Unordered set of unique space-separated tokens</span>*
     <tr>
      <th> <code data-x="">rel</code>
      <td> <code data-x="attr-link-rel">link</code>
      <td> Relationship between the document containing the <span>hyperlink</span> and the destination resource
-     <td> <span>Set of space-separated tokens</span>*
+     <td> <span>Unordered set of unique space-separated tokens</span>*
     <tr>
      <th> <code data-x="">required</code>
      <td> <code data-x="attr-input-required">input</code>;