[css-values-4] Properly handle simplification of sum nodes containing negated sum nodes #13020 (#13069)

* [css-values-4] Remove erroneous </a> tag

* [css-values-4] Properly handle simplification of sum nodes containing negated sum nodes #13020

A WPT test already exists in the form of css/css-values/calc-nesting-002.html

* ws

---------

Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: Calme1709

css-values-4/Overview.bs

@@ -4937,7 +4937,28 @@ Simplification</h4>
 				(0 - value).
 			2. If |root|'s child is a Negate node,
 				return the child's child.
-			3. Return |root|.
+			3. If |root|'s child is a Sum node:
+
+				1. Let |negated grandchildren| be an empty list
+
+				2. For each |grandchild| of the child's children:
+
+					1. If |grandchild| is a numeric value,
+						create an equivalent numeric value,
+						but with the value negated
+						(0 - value),
+						and append the result to |negated grandchildren|.
+
+					2. If |grandchild| is a Negate node
+						append |grandchild|'s child to |negated grandchildren|
+
+					3. Otherwise,
+						create a Negate node with |grandchild| as its child,
+						and append the result to |negated grandchildren|
+
+				3. Return a Sum node with |negated grandchildren| as its children
+
+			4. Return |root|.
 
 		7. If |root| is an Invert node: