[css-nesting-1] Clarify that an invalid nested style rule doesn't invalidate its parent rule. w3c#2880

Author: tabatkins

css-nesting-1/Overview.bs

@@ -274,6 +274,10 @@ Syntax {#syntax}
 	The precise details of how nested style rules are parsed
 	are defined in [[!CSS-SYNTAX-3]].
 
+	An invalid [=nested style rule=] is ignored,
+	along with its contents,
+	but does not invalidate its parent rule.
+
 	<div class="example">
 		For example, the following nestings are valid: