[css-device-adapt] Extend css-conditional rather than CSS21 to allow nesting in @media

Author: frivoal

css-device-adapt/Overview.bs

@@ -182,14 +182,14 @@ non-terminals:
       ;
 </pre>
 
-It is also added to media production to allow ''@viewport'' rules
-nested inside ''@media'' rules <span class="note">This is extending
-the CSS 2.1 syntax. A draft of CSS3 Paged Media also allows page
-inside @media.</span>:
+It is also added to the <a spec="css-conditional">nested_statement</a> production defined in [[!css-conditional]]
+to allow ''@viewport'' rules nested inside <a spec="css-conditional">conditional group rules</a>
+such as ''@media'' or ''@supports'':
 
 <pre>
-    media
-      : MEDIA_SYM S* media_list LBRACE S* [ ruleset | viewport ]* '}' S*
+    nested_statement
+      : ruleset | media | page | font_face_rule | keyframes_rule |
+        supports_rule | viewport
       ;
 </pre>