Have CSSOM be the definitive 'first' definition of the CSS interface.

Author: tabatkins

cssom/Overview.bs

@@ -3019,9 +3019,9 @@ Utility APIs {#utility-apis}
 The <code>CSS.escape()</code> Method {#the-css.escape()-method}
 ------------------------------------------------------
 
-The <code lt>CSS</code> interface is defined in CSS Conditional Rules Module. [[!CSS3-CONDITIONAL]]
+The <code lt>CSS</code> interface holds useful CSS-related functions that do not belong elsewhere.
 
-<pre class=idl>partial interface CSS {
+<pre class=idl>interface CSS {
   static DOMString escape(DOMString ident);
 };</pre>