[css-values-4] Correctly handle *either* type having a percent hint when making things consistent.

Author: tabatkins

css-values-4/Overview.bs

@@ -4356,9 +4356,11 @@ Type Checking</h3>
 			they can't be made consistent.
 			Return failure.
 
-		2. Otherwise, set |base|’s [=percent hint=]
-			to |input|’s [=percent hint=],
-			and return |base|.
+		2. If |base| has a null [=percent hint=]
+			set |base|’s [=percent hint=]
+			to |input|’s [=percent hint=].
+
+		3. Return |base|.
 	</div>
 
 	[=Math functions=] themselves have [=CSSNumericValue/types=],