[css-conditional] Add explicit ids to preserve id stability across recent preprocessor changes.

Author: dbaron

css-conditional/Overview.src.html

@@ -307,7 +307,7 @@ <h2 id="use">Placement of conditional group rules</h2>
 
 <h2 id="at-media">Media-specific style sheets:  the '@media' rule</h2>
 
-<p>The <dfn>'@media' rule</dfn> is a conditional group rule whose
+<p>The <dfn id="atmedia-rule">'@media' rule</dfn> is a conditional group rule whose
 condition is a media query.  It consists of the at-keyword
 '@media' followed by a (possibly empty) media query list (as
 defined in [[!MEDIAQ]]), followed by a group rule body.  The condition
@@ -347,7 +347,7 @@ <h2 id="at-media">Media-specific style sheets:  the '@media' rule</h2>
 
 <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
 
-<p>The <dfn>'@supports' rule</dfn> is a conditional group
+<p>The <dfn id="atsupports-rule">'@supports' rule</dfn> is a conditional group
 rule whose condition tests whether the user agent supports CSS
 property:value pairs.  Authors can use it to write style sheets that use
 new features when available but degrade gracefully when those features