Fix remaining validation error (duplicate ID CSS21) by marking CSS21 as normative in the one place where it wasn't.

Author: dbaron

css-module/Overview.html

@@ -101,7 +101,7 @@ <h2 class="no-num no-toc" id=abstract>Abstract</h2>
    contains the features of CSS level&nbsp;3 relating to <var
    class=replaceme>DESCRIBE HERE</var>. It includes and extends the
    functionality of CSS level&nbsp;2 <a href="#CSS21"
-   rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>, which builds on CSS
+   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds on CSS
    level&nbsp;1 <a href="#CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>.
    The main extensions compared to level&nbsp;2 are <var
    class=replaceme>SUMMARIZE HERE</var>.
@@ -619,16 +619,6 @@ <h3 class=no-num id=other-references>Other references</h3>
     </dd>
    <!---->
 
-   <dt id=CSS21>[CSS21]
-
-   <dd>Bert Bos; et al. <a
-    href="http://www.w3.org/TR/2011/REC-CSS2-20110607"><cite>Cascading Style
-    Sheets Level 2 Revision 1 (CSS&#160;2.1) Specification.</cite></a> 7 June
-    2011. W3C Recommendation. URL: <a
-    href="http://www.w3.org/TR/2011/REC-CSS2-20110607">http://www.w3.org/TR/2011/REC-CSS2-20110607</a>
-    </dd>
-   <!---->
-
    <dt id=CSS3COLOR>[CSS3COLOR]
 
    <dd>Tantek &#199;elik; Chris Lilley; L. David Baron. <a

css-module/Overview.src.html

@@ -66,7 +66,7 @@ <h2 class="no-num no-toc" id=abstract>Abstract</h2>
   <p>CSS is a language for describing the rendering of structured documents
   (such as HTML and XML) on screen, on paper,  in speech, etc. This module
   contains the features of CSS level&nbsp;3 relating to <var class="replaceme">DESCRIBE HERE</var>.
-  It includes and extends the functionality of CSS level&nbsp;2 [[CSS21]],
+  It includes and extends the functionality of CSS level&nbsp;2 [[!CSS21]],
   which builds on CSS level&nbsp;1 [[CSS1]].
   The main extensions compared to level&nbsp;2 are <var class="replaceme">SUMMARIZE HERE</var>.