Require UTF-8 for accept-charset

Fixes #3097.

source

@@ -42887,10 +42887,8 @@ interface <dfn>HTMLFormElement</dfn> : <span>HTMLElement</span> {
 
   <p>The <dfn><code data-x="attr-form-accept-charset">accept-charset</code></dfn> attribute gives the
   character encodings that are to be used for the submission. If specified, the value must be an
-  <span>ordered set of unique space-separated tokens</span> that are <span>ASCII
-  case-insensitive</span>, and each token must be an <span>ASCII case-insensitive</span> match for
-  one of the <span data-x="encoding label">labels</span> of an <span>ASCII-compatible
-  encoding</span>. <ref spec=ENCODING></p>
+  <span>ASCII case-insensitive</span> match for "<code data-x="">UTF-8</code>". <ref
+  spec=ENCODING></p>
 
   <p>The <dfn><code data-x="attr-form-name">name</code></dfn> attribute represents the
   <code>form</code>'s name within the <code data-x="dom-document-forms">forms</code> collection. The
@@ -118732,7 +118730,7 @@ interface <dfn>External</dfn> {
      <th> <code data-x="">accept-charset</code>
      <td> <code data-x="attr-form-accept-charset">form</code>
      <td> Character encodings to use for <span>form submission</span>
-     <td> <span>Ordered set of unique space-separated tokens</span>, <span>ASCII case-insensitive</span>, consisting of <span data-x="encoding label">labels</span> of <span data-x="ASCII-compatible encoding">ASCII-compatible encodings</span>*
+     <td> <span>ASCII case-insensitive</span> match for "<code data-x="">UTF-8</code>
     <tr>
      <th> <code data-x="">accesskey</code>
      <td> <span data-x="attr-accesskey">HTML elements</span>