[cssom-1] Make sure CSSOMString only gets dfn'd once.

Author: tabatkins

cssom-1/Overview.bs

@@ -280,7 +280,7 @@ CSSOMString {#cssomstring-type}
 Most strings in CSSOM interfaces use the <dfn interface>CSSOMString</dfn> type.
 Each implementation chooses to define it as either {{USVString}} or {{DOMString}}:
 
-<pre class=idl>
+<pre class="def lang-webidl">
 typedef USVString CSSOMString;
 </pre>