[css-overflow-3] Exclude zero-area border boxes from overflow contribution. w3c#4791

Author: fantasai

css-overflow-3/Overview.bs

@@ -223,6 +223,9 @@ Scrollable Overflow
 			Issue: Is this description of handling transforms
 			sufficiently accurate?
 
+			Border boxes with zero area
+			do not affect the [=scrollable overflow area=].
+
 		
 			The margin areas of grid item and flex item boxes
 			for which the box establishes a containing block.
@@ -238,7 +241,8 @@ Scrollable Overflow
 
 		
 			The scrollable overflow areas of all of the above boxes
-			(accounting for transforms as described above),
+			(including zero-area boxes
+			and accounting for transforms as described above),
 			provided they themselves have ''overflow: visible''
 			(i.e. do not themselves trap the overflow)
 			and that scrollable overflow is not already clipped