Export some popover dfns

These dfns need to be exported in order to be used in the fullscreen
spec: whatwg/fullscreen#204

source

@@ -81489,14 +81489,16 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
   must <span>reflect</span> the <span data-x="attr-popover">popover</span> attribute, <span>limited
   to only known values</span>.</p>
 
-  <p>Every <span data-x="HTML elements">HTML element</span> has a <dfn>popover visibility
+  <p>Every <span data-x="HTML elements">HTML element</span> has a <dfn export>popover visibility
   state</dfn>, initially <span data-x="popover-hidden-state">hidden</span>, with these potential
   values:</p>
 
   <ul>
-   <li><p><dfn data-x="popover-hidden-state">hidden</dfn></p></li>
+   <li><p><dfn export data-dfn-for="popover visibility state"
+   data-x="popover-hidden-state">hidden</dfn></p></li>
 
-   <li><p><dfn data-x="popover-showing-state">showing</dfn></p></li>
+   <li><p><dfn export data-dfn-for="popover visibility state"
+   data-x="popover-showing-state">showing</dfn></p></li>
   </ul>
 
   <p>The <code>Document</code> has an <dfn>auto popover list</dfn>, which is a <span>list</span>,