[css-lists] Fix reversed counters to count how many elements increment or set the counter (#6297)

Author: zcorpan

css-lists-3/Overview.bs

@@ -1142,7 +1142,29 @@ Instantiating Counters</h4>
 
 	When a [=counter=] is [=instantiated=] without an initial value,
 	the user agent must dynamically calculate the initial value at layout-time
-	to be the number of elements and pseudo-elements that [=instantiate=] the same counter in the same [=scope=], plus 1.
+	to be the value returned by the following algorithm:
+
+	1. Let |num| be 0.
+
+	2. Let |first| be true.
+
+	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.
+
+		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|.
 
 	Note: Only [=reversed=] counters can be instantiated without an initial value.