[css-values-4][css-sizing-3] Define 'contains a percentage', ref it in Sizing. w3c#10017

Author: tabatkins

css-sizing-3/Overview.bs

@@ -1063,7 +1063,7 @@ Intrinsic Contributions of Percentage-Sized Boxes</h4>
 		then the entire value of any
 		[=max size property=] or [=preferred size property=]
 		('width'/'max-width'/'height'/'max-height')
-		specified as an expression containing a percentage
+		specified as an expression [=containing a percentage=]
 		(such as ''10%'' or ''calc(10px + 0%)'')
 		that is <a lt="cyclic percentage">cyclic</a>
 		is treated

css-values-4/Overview.bs

@@ -4336,6 +4336,12 @@ Type Checking</h3>
 			</dl>
 	</div>
 
+	<div algorithm>
+		A value <dfn export for=CSS lt="contain a percentage|percentage-containing">contains a percentage</dfn>
+		if its type is [=CSSNumericValue/type=] is «[ "percent" → 1 ]»,
+		or its type's [=percent hint=] is non-null.
+	</div>
+
 	<div algorithm>
 		Two or more calculations have a <dfn export for=CSS local-lt="consistent">consistent type</dfn>
 		if [=add two types/adding the types=] doesn't result in failure.