Add selectedcontent element and allowances for 'button' part of select (#528)

* Add <selectedoption> element
* updates selectedcontent and button element
* update selectedcontent role allowances
account for misuse of element - where the element will just be a generic element and thus is name prohibited, but any role/attributes can be used to modify/fix the errant element, as needed.
* changelog update

---------

Co-authored-by: Scott O'Hara <[email protected]>

Author: josepharhar

index.html

@@ -64,6 +64,12 @@
         the following substantive additions and/or corrections have been proposed:
       </p>
       <ul>
+        <li>
+          <a href="https://github.com/w3c/html-aria/pull/528">23 July 2025 - Addition:</a>
+          Add the <a href="#el-selectedcontent">`selectedcontent`</a> element and provide updated
+          allowances for the <a href="#el-button">`button`</a> element when it is used in the context
+          of a customized `select` element.
+        </li>
         <li>
           <a href="https://github.com/w3c/html-aria/pull/550">23 July 2025 - Correction:</a>
           Clarify that the <a href="#el-html">`html`</a> element is a `generic` element, and that
@@ -893,7 +899,8 @@ <h2 id="docconformance">
               [^button^]
             </th>
             <td>
-              <code>role=<a href="#index-aria-button">button</a></code>
+              <p><code>role=<a href="#index-aria-button">button</a></code></p>
+              <p>If the `button` is the first child of a `select` element, the element is `inert`.</p>
             </td>
             <td>
               <p>
@@ -919,6 +926,9 @@ <h2 id="docconformance">
                 <a data-cite="wai-aria-1.2#global_states">Global `aria-*` attributes</a> 
                 and any `aria-*` attributes applicable to the allowed roles.
               </p>
+              <hr>
+              <p>If the `button` is the first child of a `select` element:
+                <strong class="nosupport"><a>No `role`</a> or `aria-*` attributes</strong></p>
             </td>
           </tr>
           <tr>
@@ -2954,6 +2964,28 @@ <h2 id="docconformance">
               </p>
             </td>
           </tr>
+          <tr>
+            <th id="el-selectedcontent" tabindex="-1">
+              [^selectedcontent^]
+            </th>
+            <td>
+              <code>role=<a href="#index-aria-generic">generic</a></code>
+            </td>
+            <td>
+              <p>
+                If used as a valid descendant of a `select` element: <strong class="nosupport"><a>no `role`</a> or `aria-*` attributes</strong>
+              </p>
+              <p>
+                Otherwise, <a><strong>any `role`</strong></a> if the element is used outside of its intended context as a child of the `button` part of a customizable `select` element, 
+                though <code><a href="#index-aria-generic">generic</a></code> is NOT RECOMMENDED.
+              </p>
+              <p><a>Naming Prohibited</a></p>
+              <p>
+                Otherwise, <a data-cite="wai-aria-1.2#global_states">global `aria-*` attributes</a> 
+                and any `aria-*` attributes applicable to the allowed roles.
+              </p>
+            </td>
+          </tr>
           <tr>
             <th id="el-slot" tabindex="-1">
               [^slot^]