Make <link>'s as="" attribute case-insensitive

Closes whatwg#1665 by aligning with other enumerated attributes.

source

@@ -6910,13 +6910,6 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   data-x="idl-USVString">USVString</code> attribute but doesn't fall into any of the above
   categories, then the getting and setting must be done in a transparent, case-preserving manner.</p>
 
-  <p>If a reflecting IDL attribute is an <span>IDL enumeration</span> attribute, then, on getting,
-  if the corresponding content attribute's value <span
-  data-x="case-sensitive">case-sensitively</span> matches one of the enumerated values, then the IDL
-  attribute must return the content attribute's value; otherwise it must return the content
-  attribute's default value. On setting, the content attribute must be set to the specified new
-  value.</p>
-
   <p>If a reflecting IDL attribute is a <code data-x="idl-boolean">boolean</code> attribute, then on
   getting the IDL attribute must return true if the content attribute is set, and false if it is
   absent. On setting, the content attribute must be removed if the IDL attribute is set to false,
@@ -12848,7 +12841,7 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   [<span>CEReactions</span>] attribute USVString <span data-x="dom-link-href">href</span>;
   [<span>CEReactions</span>] attribute DOMString? <span data-x="dom-link-crossOrigin">crossOrigin</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-link-rel">rel</span>;
-  [<span>CEReactions</span>] attribute <span>RequestDestination</span> <span data-x="dom-link-as">as</span>; // (default "")
+  [<span>CEReactions</span>] attribute DOMString <span data-x="dom-link-as">as</span>; // (default "")
   [SameObject, PutForwards=<span data-x="dom-DOMTokenList-value">value</span>] readonly attribute <span>DOMTokenList</span> <span data-x="dom-link-relList">relList</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-link-media">media</span>;
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-link-nonce">nonce</span>;
@@ -13043,11 +13036,13 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
 
   <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. If the attribute is present, the
-  value must <span data-x="case-sensitive">case-sensitively</span> match one of the <span
-  data-x="concept-request-destination">destination keywords</span>. The default value is the empty
-  string. 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
+  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>
@@ -13095,7 +13090,6 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   <hr>
 
   <p>The IDL attributes
-  <dfn><code data-x="dom-link-as">as</code></dfn>,
   <dfn><code data-x="dom-link-href">href</code></dfn>,
   <dfn><code data-x="dom-link-hreflang">hreflang</code></dfn>,
   <dfn><code data-x="dom-link-integrity">integrity</code></dfn>,
@@ -13110,6 +13104,10 @@ interface <dfn>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   <p class="note">There is no reflecting IDL attribute for the <code
   data-x="attr-link-color">color</code> attribute, but this might be added later.</p>
 
+  <p>The <dfn><code data-x="dom-link-as">as</code></dfn> IDL attribute must <span>reflect</span> the
+  <code data-x="attr-link-as">as</code> content attribute, <span>limited to only known
+  values</span>.</p>
+
   <p>The <dfn><code data-x="dom-link-crossOrigin">crossOrigin</code></dfn> IDL attribute must
   <span>reflect</span> the <code data-x="attr-link-crossorigin">crossorigin</code> content
   attribute.</p>