[css-syntax] Fix the definition of <declaration-rule-list>, and correct some examples. w3c#8834

Author: tabatkins

css-syntax-3/Overview.bs

@@ -3469,12 +3469,19 @@ Defining Block Contents: the <<block-contents>>, <<declaration-list>>, <<qualifi
 	the following productions may be used
 
 	* <dfn>&lt;declaration-list></dfn>:
+		only [=declarations=] are allowed;
 		[=at-rules=] and [=qualified rules=] are automatically invalid.
 	* <dfn>&lt;qualified-rule-list></dfn>:
+		only [=qualified rules=] are allowed;
 		[=declarations=] and [=at-rules=] are automatically invalid.
+	* <dfn>&lt;at-rule-list></dfn>:
+		only [=at-rules=] are allowed;
+		[=declarations=] and [=qualified rules=] are automatically invalid.
 	* <dfn>&lt;declaration-rule-list></dfn>:
-		[=at-rules=] are automatically invalid.
+		[=declarations=] and [=at-rules=] are allowed;
+		[=qualified-rules=] are automatically invalid.
 	* <dfn>&lt;rule-list></dfn>:
+		[=qualified rules=] and [=at-rules=] are allowed;
 		[=declarations=] are automatically invalid.
 
 	All of these are exactly equivalent to <<block-contents>> in terms of parsing,
@@ -3489,7 +3496,7 @@ Defining Block Contents: the <<block-contents>>, <<declaration-list>>, <<qualifi
 		* [=Style rules=] use <<block-contents>>.
 		* ''@font-face'' uses <<declaration-list>>.
 		* ''@page'' uses <<declaration-rule-list>>.
-		* ''@keyframes'' uses <<rule-list>>
+		* ''@keyframes'' uses <<qualified-rule-list>>
 	</div>
 
 	<div class=example>
@@ -3503,7 +3510,7 @@ Defining Block Contents: the <<block-contents>>, <<declaration-list>>, <<qualifi
 		The grammar for ''@keyframes'' can be written as:
 
 		<pre>
-			<<@keyframes>> = @keyframes { <<rule-list>> }
+			<<@keyframes>> = @keyframes { <<qualified-rule-list>> }
 			<<keyframe-rule>> = <<keyframe-selector>> { <<declaration-list>> }
 		</pre>