[css-values-4] Define stacked multipliers w3c#6085

Author: fantasai

css-values-4/Overview.bs

@@ -262,6 +262,13 @@ Component Value Multipliers</h3>
 			at least one component value must not be omitted.
 	</ul>
 
+	The [=+=] and [=#=] multipliers may be stacked as ''+#'';
+	similarly, the [=#=] and [=?=] multipliers may be stacked as ''#?''.
+	These stacks each represent the later multiplier
+	applied to the result of the earlier multiplier.
+	(These same stacks can be represented using grouping,
+	but in complex grammars this can push the number of brackets beyond readability.)
+
 	For repeated component values (indicated by <css>*</css>, <css>+</css>, or <css>#</css>),
 	UAs must support at least 20 repetitions of the component.
 	If a property value contains more than the supported number of repetitions,