Addition: selectedoption element

I recreated the [original PR](w3c/html-aam#566) by @josepharhar

The `<selectedoption>` element is part of the [customizable select feature](whatwg/html#9799) and is being added to HTML [here](whatwg/html#10633).

## Implementation

* WPT tests: web-platform-tests/wpt#45096
* Implementations (link to issue or when done, link to commit):
   * WebKit: TODO
   * Gecko: TODO
   * Blink: https://chromium.googlesource.com/chromium/src/+/18b5eac27b14b409503aa8047cf9358082a0e0df

Co-authored-by: Joey Arhar @josepharhar

html-aam/index.html

@@ -6343,6 +6343,56 @@ <h4 id="el-select-combobox">`select`<span class="el-context"> (with NO `multiple
             </tr>
           </tbody>
         </table>
+        <h4 id="el-selectedoption">`selectedoption`</h4>
+        <table aria-labelledby="el-selectedoption">
+          <tbody>
+            <tr>
+              <th>HTML Specification</th>
+              <td>
+                <a data-cite="HTML">`selectedoption`</a>
+              </td>
+            </tr>
+            <tr>
+              <th>[[wai-aria-1.2]]</th>
+              <td>??? <!-- todo: not mapped, html-selectedoption, generic, or none? --></td>
+            </tr>
+            <tr>
+              <th><a data-cite="core-aam-1.2/#roleMappingComputedRole">Computed Role</a></th>
+              <td class="role-computed"><div class="general">Use WAI-ARIA mapping</div></td>
+            </tr>
+            <tr>
+              <th>
+                <a href="https://msdn.microsoft.com/en-us/library/dd373608%28v=VS.85%29.aspx">MSAA</a> + <a href="http://accessibility.linuxfoundation.org/a11yspecs/ia2/docs/html/">IAccessible2</a>
+              </th>
+              <td>
+                <div class="general">???</div>
+              </td>
+            </tr>
+            <tr>
+              <th><a href="https://msdn.microsoft.com/en-us/library/ms726297%28v=VS.85%29.aspx">UIA</a></th>
+              <td>
+                <div class="general">???</div>
+              </td>
+            </tr>
+            <tr>
+              <th><a href="https://gnome.pages.gitlab.gnome.org/atk/">ATK</a></th>
+              <td>
+                <div class="general">???</div>
+              </td>
+            </tr>
+            <tr>
+              <th><a href="https://developer.apple.com/reference/appkit/nsaccessibility">AX</a></th>
+              <td>
+                <div class="general">???</div>
+              </td>
+            </tr>
+            <!-- <th><a href="#accessible-name-and-description-computation">Naming Algorithm</a></th> -->
+            <tr>
+              <th>Comments</th>
+              <td></td>
+            </tr>
+          </tbody>
+        </table>
         <h4 id="el-slot">`slot`</h4>
         <table aria-labelledby="el-slot">
           <tbody>