[css-counter-styles] Clarify that making the entire counter style invalid also aborts an attribute setter.

Author: tabatkins

css-counter-styles/Overview.bs

@@ -2058,7 +2058,9 @@ The <code>CSSCounterStyleRule</code> interface</h3>
 
 				<li>
 					If the returned value is invalid according to the given descriptor's grammar,
+					or would cause the ''@counter-style'' rule to become invalid,
 					do nothing and abort these steps.
+					(For example, some systems require the 'symbols' descriptor to contain two values.)
 
 				<li>
 					If the attribute being set is <a idl>system</a>,

css-counter-styles/Overview.html

@@ -2078,7 +2078,9 @@ <h3 class="heading settled heading" data-level=9.2 id=the-csscounterstylerule-in
 
 				<li>
 					If the returned value is invalid according to the given descriptor’s grammar,
+					or would cause the <a class=css data-link-type=maybe href=#at-ruledef-counter-style title=@counter-style>@counter-style</a> rule to become invalid,
 					do nothing and abort these steps.
+					(For example, some systems require the <a class=property data-link-type=propdesc href=#descdef-symbols title=symbols>symbols</a> descriptor to contain two values.)
 
 				<li>
 					If the attribute being set is <a data-link-type=idl href=#dom-csscounterstylerule-system title=system>system</a>,