[css-values-4] Per WG resolution, explicitly drop min/max() when there's only one child left. w3c#9559

Author: tabatkins

css-values-4/Overview.bs

@@ -4551,7 +4551,10 @@ Simplification</h4>
 				and replace |child| with the result,
 				removing all other child nodes involved.
 
-			2. Return |root|.
+			2. If |root| has only one child,
+				return the child.
+
+				Otherwise, return |root|.
 
 		6. If |root| is a Negate node: