[css-values] Use the new 'invert a type' algo.

Author: tabatkins

css-values-4/Overview.bs

@@ -1774,10 +1774,11 @@ Type Checking</h4>
 		Otherwise, the sub-expression's [=CSSNumericValue/type=] is the returned type.
 
 	* At a <css>/</css> sub-expression,
-		find the [=CSSNumericValue/types=] of its left and right arguments,
-		then negate the values of every [=map/entry=] in the right argument’s [=CSSNumericValue/type=].
-		Attempt to [=multiply two types|multiply=] the left argument’s type
-		and the negated right argument’s type.
+		let |left type| be the result of finding the [=CSSNumericValue/types=] of its left argument,
+		and |right type| be the result of finding the [=CSSNumericValue/types=] of its right argument
+		and then [=invert a type|inverting=] it.
+
+		Attempt to [=multiply two types|multiply=] the |left type| and |right type|.
 		If this returns failure,
 		the entire [=math function=] is invalid.
 		Otherwise, the sub-expression's [=CSSNumericValue/type=] is the returned type.