Add userAgent capability

index.html

@@ -1569,6 +1569,13 @@ <h2>Capabilities</h2>
   <td>Describes the <a>current session</a>’s <a>user prompt handler</a>.
    Defaults to the <a>dismiss and notify state</a>.
  </tr>
+
+ <tr>
+  <td><dfn>User Agent</dfn>
+  <td>"<code>userAgent</code>"
+  <td>string
+  <td>Provides the <a>remote end</a>'s <a>default User-Agent value</a>.
+ </tr>
 </table>
 
 <section>
@@ -1992,6 +1999,9 @@ <h3>Processing capabilities</h3>
 
    <dt>"<code>setWindowRect</code>"
    <dd>Boolean indicating whether the <a>remote end</a> supports all of the <a href=#resizing-and-positioning-windows>resizing and positioning</a> <a>commands</a>.
+
+   <dt>"<code>userAgent</code>"
+   <dd>String containing the <a>default User-Agent value</a>
   </dl>
 
  <li><p>Optionally add <a>extension capabilities</a> as entries
@@ -11103,6 +11113,7 @@ <h2>Index</h2>
  <dd><p>The following terms are defined in the WHATWG Fetch specification: [[FETCH]]
   <ul>
    <!-- Body --> <li><dfn><a href=https://fetch.spec.whatwg.org/#concept-request-body>Body</a></dfn>
+   <!-- default User-Agent value --> <li><dfn><a href=https://fetch.spec.whatwg.org/#default-user-agent-value>default User-Agent value</a></dfn>
    <!-- Header --> <li><dfn><a href="https://fetch.spec.whatwg.org/#concept-header">Header</a></dfn>
    <!-- Header Name --> <li><dfn><a href="https://fetch.spec.whatwg.org/#concept-header-name">Header Name</a></dfn>
    <!-- Header Value --> <li><dfn><a href="https://fetch.spec.whatwg.org/#concept-header-value">Header Value</a></dfn>