[css-values-5] Make the attr() substitution algo properly handle the legacy fallback behavior. #11818

Author: tabatkins

css-values-5/Overview.bs

@@ -1716,9 +1716,11 @@ Attribute Value Substitution: the ''attr()'' notation</h4>
 			Let |fallback| be the <<declaration-value>>? argument specified in the function,
 			or the [=guaranteed-invalid value=] if it was omitted.
 
-		2. If there is no attribute named |attr name| on |el|,
-			return the [=guaranteed-invalid value=] and |fallback|.
-			Otherwise, let |attr value| be that attribute's value.
+		2. Let |attr value| be the value of the |attr name| attribute on |el|.
+
+			If such an attribute does not exist,
+			return an empty CSS string if |syntax| is null,
+			or the [=guaranteed-invalid value=] and |fallback| otherwise.
 
 		3. If |syntax| is null,
 			return a CSS <<string>>