[cssom] Work around a bug in the build script

Author: Simon Pieters

cssom/Overview.html

@@ -2482,6 +2482,8 @@ <h3 id="extensions-to-the-element-interface"><span class="secno">7.4 </span>Exte
   <!-- ........................................................................................................................ -->
 
   <h3 id="the-pseudoelement-interface"><span class="secno">7.5 </span>The <code>PseudoElement</code> Interface</h3>
+  
+  <pre class="idl">PseudoElement implements GetStyleUtils;</pre><!-- XXX remove when https://www.w3.org/Bugs/Public/show_bug.cgi?id=21973 is fixed -->
 
   <!--ATRISK--><p>The <code>PseudoElement</code> interface is used for representing CSS pseudo-elements. A <code>PseudoElement</code> object is associated with an <code>Element</code> object.</p>
 <pre class="idl"><span class="idlInterface" id="widl-def-PseudoElement">interface <span class="idlInterfaceID">PseudoElement</span> {

cssom/cssom-source

@@ -1955,6 +1955,8 @@
   <!-- ........................................................................................................................ -->
 
   <h3>The <code>PseudoElement</code> Interface</h3>
+  
+  <pre class=idl>PseudoElement implements GetStyleUtils;</pre><!-- XXX remove when https://www.w3.org/Bugs/Public/show_bug.cgi?id=21973 is fixed -->
 
   <!--{@idl(PseudoElement)}-->