[css-position-3] Specify the Chrome behavior for inline CBs when the edges are misordered, per WG resolution. w3c#609

Author: tabatkins

css-position-3/Overview.bs

@@ -329,8 +329,13 @@ Containing Blocks of Positioned Boxes</h3>
 					take the [=start=]most/[=end=]most fragment
 					as the first/last fragment.
 
-					Note: If the ancestor breaks across a line,
-					the “start” position might more end-ward than the “end” position.
+					However, since a [=containing block=] cannot have negative area:
+					if the [=block-end=] edge would be [=start=]wards of the [=block-start=] edge,
+					then the [=block-end=] edge of the [=containing block=]
+					instead uses the [=block-start=] edge;
+					and analogously for the [=inline axis=].
+					(Thus the [=containing block=] will have zero height and/or width as a result.)
+
 			</dl>
 
 			If no ancestor establishes one,