[css-lists-3] Update algorithm for initial counter value in reversed list (#13513)

Author: jja08111

css-lists-3/Overview.bs

@@ -1148,25 +1148,26 @@ Instantiating Counters</h4>
 
 	1. Let |num| be 0.
 
-	2. Let |first| be true.
+	2. Let |lastNonZeroIncrementNegated| be 0.
 
 	3. For each element or pseudo-element |el| that increments or
 		sets the same counter in the same [=scope=]:
 
 		1. Let |incrementNegated| be |el|'s 'counter-increment' integer value for this counter,
 			multiplied by -1.
 
-		2. If |first| is true,
-			then add |incrementNegated| to |num| and
-			set |first| to false.
+		2. If |incrementNegated| is not zero,
+			then set |lastNonZeroIncrementNegated| to |incrementNegated|.
 
 		3. If |el| sets this counter with 'counter-set',
 			then add that integer value to |num| and
 			break this loop.
 
 		4. Add |incrementNegated| to |num|.
 
-	4. Return |num|.
+	4. Add |lastNonZeroIncrementNegated| to |num|.
+
+	5. Return |num|.
 
 	Note: Only [=reversed=] counters can be instantiated without an initial value.