[css-position-3] Specify that auto margins are treated as zero for calculating static position. w3c#665

Author: fantasai

css-position-3/Overview.bs

@@ -757,6 +757,8 @@ Resolving Automatic Insets</h4>
 	is the [=initial containing block=] instead of the [=viewport=],
 	and all [=scroll containers=] should be assumed
 	to be scrolled to their [=initial scroll position=].
+	Additionally, all ''margin/auto'' margins on the box itself
+	are treated as zero.
 
 <h4 id="abspos-breaking" oldids="breaking">
 Fragmenting Absolutely-positioned Elements</h4>