Remove more XML-derived attribute name validation

* Change the behavior of the dataset named setter.
* Change the conformance-related definitions for custom data attributes and custom element attributes.

Closes #11439.

Author: domenic

source

@@ -1744,10 +1744,6 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   type <code>button</code>, meaning they have the local name "<code data-x="">button</code>" and
   (implicitly as defined above) the <span>HTML namespace</span>.</p>
 
-  <p>Attribute names are said to be <dfn>XML-compatible</dfn> if they match the <code
-  data-x="xml-Name">Name</code> production defined in XML and they contain no U+003A COLON
-  characters (:). <ref>XML</ref></p>
-
 
   <h4>DOM trees</h4>
 
@@ -3345,6 +3341,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#abortcontroller-signal-abort">signal abort</dfn></li>
      <li><dfn data-x="AbortSignal-add" data-x-href="https://dom.spec.whatwg.org/#abortsignal-add">add</dfn></li>
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-element-attributes-get-by-name">get an attribute by name</dfn> algorithm</li>
+     <li><dfn data-x-href="https://dom.spec.whatwg.org/#valid-attribute-local-name">valid attribute local name</dfn></li>
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#valid-element-local-name">valid element local name</dfn></li>
     </ul>
 
@@ -14242,8 +14239,8 @@ background: transparent">blue&lt;/span>.&lt;/p></code></pre>
 
   <p>A <dfn>custom data attribute</dfn> is an attribute in no namespace whose name starts with the
   string "<dfn><code data-x="attr-data-*">data-</code></dfn>", has at least one character after the
-  hyphen, is <span>XML-compatible</span>, and contains no <span data-x="ASCII upper alpha">ASCII
-  upper alphas</span>.</p>
+  hyphen, is a <span>valid attribute local name</span>, and contains no <span
+  data-x="ASCII upper alpha">ASCII upper alphas</span>.</p>
 
   <p class="note">All attribute names on <span>HTML elements</span> in <span>HTML documents</span>
   get ASCII-lowercased automatically, so the restriction on ASCII uppercase letters doesn't affect
@@ -14429,8 +14426,8 @@ interface <dfn interface>DOMStringMap</dfn> {
 
    <li><p>Insert the string <code data-x="">data-</code> at the front of <var>name</var>.</p></li>
 
-   <li><p>If <var>name</var> does not match the XML <code data-x="xml-Name">Name</code> production,
-   throw an <span>"<code>InvalidCharacterError</code>"</span> <code>DOMException</code>.</p></li>
+   <li><p>If <var>name</var> is not a <span>valid attribute local name</span>, then throw an
+   <span>"<code>InvalidCharacterError</code>"</span> <code>DOMException</code>.</p></li>
 
    <li><p><span data-x="concept-element-attributes-set-value">Set an attribute value</span> for the
    <code>DOMStringMap</code>'s <span data-x="concept-DOMStringMap-element">associated element</span>
@@ -73326,10 +73323,10 @@ document.body.append(parent);
 
   <p>Any namespace-less attribute that is relevant to the element's functioning, as determined by
   the element's author, may be specified on an <span>autonomous custom element</span>, so long as
-  the attribute name is <span>XML-compatible</span> and contains no <span data-x="ASCII upper
-  alpha">ASCII upper alphas</span>. The exception is the <code data-x="attr-is">is</code> attribute,
-  which must not be specified on an <span>autonomous custom element</span> (and which will have no
-  effect if it is).</p>
+  the attribute name is a <span>valid attribute local name</span> and contains no <span
+  data-x="ASCII upper alpha">ASCII upper alphas</span>. The exception is the <code
+  data-x="attr-is">is</code> attribute, which must not be specified on an <span>autonomous custom
+  element</span> (and which will have no effect if it is).</p>
 
   <p><span data-x="customized built-in element">Customized built-in elements</span> follow the
   normal requirements for attributes, based on the elements they extend. To add custom