[css-overflow-3] Handle negative margins.

Author: fantasai

css-overflow-3/Overview.bs

@@ -146,11 +146,12 @@ Scrollable overflow</h3>
 	the minimal rectangle whose axis is aligned to the box's axes
 	and contains the <a>scrollable overflow region</a>.
 
-	The <a>scrollable overflow region</a> of a box includes:
+	The <a>scrollable overflow region</a> is the union of:
 	<ul>
 		<li>the box’s own content and padding areas
 		<li>all line boxes directly contained by the box
-		<li>the margin boxes of all in-flow and floated boxes
+		<li>the margin boxes <em>and</em> border boxes
+		    of all in-flow and floated boxes
 		    for which it is the containing block
 		    (accounting for transforms as described below).
 		<li>the padding boxes of all absolutely-positioned boxes