[css-sizing] Specify that auto margins are treated as zero for intrinsic sizing purposes. https://lists.w3.org/Archives/Public/www-style/2015Mar/0187.html

Author: fantasai

css-sizing-3/Overview.bs

@@ -212,6 +212,9 @@ Intrinsic Size Contributions</h3>
 			The size that a box contributes to its <a>containing block</a>'s <a>min-content size</a>.
 	</dl>
 
+	Intrinsic size contributions are based on the <a>outer size</a> of the box;
+	for this purpose ''auto'' margins are treated as zero.
+
 <h3 id='constraints'>
 Intrinsic Size Constraints</h3>
 

css-sizing/Overview.bs

@@ -212,6 +212,9 @@ Intrinsic Size Contributions</h3>
 			The size that a box contributes to its <a>containing block</a>'s <a>min-content size</a>.
 	</dl>
 
+	Intrinsic size contributions are based on the <a>outer size</a> of the box;
+	for this purpose ''auto'' margins are treated as zero.
+
 <h3 id='constraints'>
 Intrinsic Size Constraints</h3>