diff --git a/css-values-4/Overview.bs b/css-values-4/Overview.bs
index e575d5ce41f..47d5e153747 100644
--- a/css-values-4/Overview.bs
+++ b/css-values-4/Overview.bs
@@ -1153,7 +1153,7 @@ URL Processing Model</h4>
 			1. If |cssRuleOrDeclaration| is a [=CSS declaration block=] whose [=parent CSS rule=] is not null,
 				set |cssRuleOrDeclaration| to |cssRuleOrDeclaration|'s [=parent CSS rule=].
 
-			1. If |cssRuleOrDeclaration| is a [=CSS rule=], set |sheet| to |cssRuleOrDeclaration|'s {{CSSRule/parentStyleSheet|parent style sheet}}</a>.
+			1. If |cssRuleOrDeclaration| is a [=CSS rule=], set |sheet| to |cssRuleOrDeclaration|'s {{CSSRule/parentStyleSheet|parent style sheet}}.
 
 			1. If |sheet| is not null:
 				1. If |sheet|'s <a spec=cssom>stylesheet base URL</a> is not null,
@@ -4906,7 +4906,28 @@ 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: