[css-conditiona-3] Clean up placement prose. w3c#5697

Author: fantasai

css-conditional-3/Overview.bs

@@ -172,16 +172,17 @@ The user agent will consult the namespace map to see whether a namespace url exi
 <h2 id="use">
 Placement of conditional group rules</h2>
 
-Conditional group rules are allowed at the top-level of a style
-sheet, and inside other conditional group rules.  CSS processors
-<strong>must</strong> process such rules as <a
-href="#processing">described above</a>.
-
-Any rules that are not allowed after a style rule (e.g., ''@charset'',
-''@import'', or ''@namespace'' rules) are also not allowed after a
-conditional group rule.  Therefore, style sheets <strong>must
-not</strong> place such rules after a conditional group rule, and CSS
-processors <strong>must</strong> ignore such rules.
+Conditional group rules are allowed
+wherever [=style rules=] are allowed
+(at the top-level of a style sheet,
+as well as within other conditional group rules).
+CSS processors <strong>must</strong> process such rules
+as <a href="#processing">described above</a>.
+
+Any [=at-rules=] that are not allowed after a style rule
+(e.g., ''@charset'', ''@import'', or ''@namespace'' rules)
+are also not allowed after a conditional group rule,
+and are therefore [=invalid=] when so placed.
 
 <h2 id="at-media">
 Media-specific style sheets:  the ''@media'' rule</h2>