[css-cascade] Rephrase the rule for placement of @import, make it clear that misplacing it makes the rule invalid.

--HG--
extra : rebase_source : 90e400fb406d0657a2ffe6c996ed28e5fb40254d

Author: tabatkins

css-cascade/Overview.html

@@ -265,9 +265,11 @@ <h2 id=at-import><span class=secno>2. </span> Importing style sheets: the
 
   <p> The <dfn id=atimport>‘<code class=css>@import</code>’</dfn> rule
    allows users to import style rules from other style sheets. Any ‘<a
-   href="#atimport"><code class=css>@import</code></a>’ rules must follow
-   all ‘<code class=css>@charset</code>’ rules and precede all other
-   at-rules and style rules in a style sheet. The ‘<a
+   href="#atimport"><code class=css>@import</code></a>’ rules must precede
+   all other at-rules and style rules in a stylesheet (besides ‘<code
+   class=css>@charset</code>’, which must be the first thing in the
+   stylesheet if it exists), or else the ‘<a href="#atimport"><code
+   class=css>@import</code></a>’ rule is invalid. The ‘<a
    href="#atimport"><code class=css>@import</code></a>’ keyword must be
    followed by the URI of the style sheet to include. A string is also
    allowed; it will be interpreted as if it had url(…) around it.

css-cascade/Overview.src.html

@@ -96,8 +96,9 @@ <h2 id="at-import">
 
 	<p>
 		The <dfn>''@import''</dfn> rule allows users to import style rules from other style sheets.	
-		Any ''@import'' rules must follow all ''@charset'' rules 
-		and precede all other at-rules and style rules in a style sheet.	
+		Any ''@import'' rules must precede all other at-rules and style rules in a stylesheet
+		(besides ''@charset'', which must be the first thing in the stylesheet if it exists),
+		or else the ''@import'' rule is invalid.
 		The ''@import'' keyword must be followed by the URI of the style sheet to include. 
 		A string is also allowed; 
 		it will be interpreted as if it had url(&hellip;) around it.