[css-conditional] Use <span id> rather than <a id> for compatibility anchor to confuse bikeshed less (and fix LINK ERROR).

Author: dbaron

css-conditional-3/Overview.bs

@@ -997,7 +997,7 @@ interface CSSDocumentRule : CSSConditionRule {
 
 
 <h3 id='the-css-namespace'>
-<a id='the-css-interface'>The <code>CSS</code> namespace, and the <code title=''>supports()</code> function</a></h3>
+<span id='the-css-interface'>The <code>CSS</code> namespace, and the <code title=''>supports()</code> function</span></h3>
 
 The {{CSS}} namespace holds useful CSS-related functions that do not belong elsewhere.