[css-flexbox] Per #665, specify that auto margins are treated as zero when finding the static position. (This statement should be generically applied in css-position, but that spec isn't maintained atm.)

Author: fantasai

css-flexbox/Overview.bs

@@ -675,7 +675,8 @@ Absolutely-Positioned Flex Children</h3>
 	assuming both the child and the flex container
 	were fixed-size boxes of their used size.
 	For this purpose,
-	a value of ''align-self: auto'' is treated identically to ''align-self/start''.
+	a value of ''align-self: auto'' is treated identically to ''align-self/start'',
+	and ''margin/auto'' margins are treated as zero.
 
 	<div class="note">
 		In other words,
@@ -3430,6 +3431,15 @@ Substantive Changes and Bugfixes</h4>
 				were all part of the table box's border+padding area,
 				and the table box were the <a>flex item</a>.</p></ins>
 			</blockquote>
+		<li id="change-2016-auto-margin-abspos">
+			Clarified that auto margins are treated as zero
+			for the purpose of calculating a absolutely-positioned flex container child’s static position.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/665">Issue 665</a>)
+			<blockquote>
+				<p>For this purpose,
+				a value of ''align-self: auto'' is treated identically to ''align-self/start''<ins>,
+				and ''margin/auto'' margins are treated as zero</ins>.</p>
+			</blockquote>
 	</ul>
 
 <h4 id="change-201605-clarify">