[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</h3>
 			Issue: Is this description of handling transforms
 			sufficiently accurate?
 
+			Border boxes with zero area
+			do not affect the [=scrollable overflow area=].
+
 		<li>
 			The margin areas of <a>grid item</a> and <a>flex item</a> boxes
 			for which the box establishes a containing block.
@@ -238,7 +241,8 @@ Scrollable Overflow</h3>
 
 		<li>
 			The <a>scrollable overflow areas</a> 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 <a>scrollable overflow</a> is not already clipped