[css-values-4] Clarify that zero terms cannot be removed. #3741

See #3741 (comment)
w3c · Apr 6, 2023 · 72d9aca · 72d9aca
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commit 72d9aca
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diff --git a/css-values-4/Overview.bs b/css-values-4/Overview.bs
@@ -4548,6 +4548,11 @@ Simplification</h4>
 				return the child.
 				Otherwise, return |root|.
 
+			Note: Zero-valued terms cannot be simply removed from a Sum;
+			they can only be combined with other values that have identical units.
+			(This is because the mere presence of a unit, even with a zero value,
+			can sometimes imply a change in behavior.)
+
 		9. If |root| is a Product node:
 
 			1. For each of |root|'s children