Meta: use XXX as issue class

Author: annevk

index.bs

@@ -5391,7 +5391,7 @@ defined in this specification are applicable to [=enumerations=].
 
 <h3 id="idl-callback-functions">Callback functions</h3>
 
-<p class="issue">
+<p class="XXX">
     The “Custom DOM Elements” spec wants to use callback function types for
     platform object provided functions.  Should we rename “callback functions” to
     just “functions” to make it clear that they can be used for both purposes?