[css-cascade] Simplify @charset parenthetical. w3c#5313

Author: fantasai

css-cascade-3/Overview.bs

@@ -92,7 +92,7 @@ Importing Style Sheets: the ''@import'' rule</h2>
 		as if they were written literally into the stylesheet at the point of the ''@import''.
 
 	Any ''@import'' rules must precede all other valid at-rules and style rules in a style sheet
-	(with ''@charset'' counting as an invalid rule), or else the ''@import'' rule is invalid.
+	(ignoring ''@charset''), or else the ''@import'' rule is invalid.
 	The syntax of ''@import'' is:
 
 	<pre class='prod'>@import [ <<url>> | <<string>> ] <<media-query-list>>? ;</pre>

css-cascade-4/Overview.bs

@@ -98,7 +98,7 @@ Importing Style Sheets: the ''@import'' rule</h2>
 		as if they were written literally into the stylesheet at the point of the ''@import''.
 
 	Any ''@import'' rules must precede all other valid at-rules and style rules in a style sheet
-	(with ''@charset'' counting as an invalid rule), or else the ''@import'' rule is invalid.
+	(ignoring ''@charset''), or else the ''@import'' rule is invalid.
 	The syntax of ''@import'' is:
 
 	<pre class='prod'>