[css-sizing] Define intrinsic size contribution of percentage margins and paddings as zero per WG resolution. Fixes w3c#347.

Author: fantasai

css-sizing-3/Overview.bs

@@ -521,14 +521,20 @@ Stretch-fit Sizing</h3>
 	* Otherwise, the percentage is resolved against the containing block’s size.
 		(The containing block’s size is not re-resolved based on the resulting size of the box;
 		the contents might thus overflow or underflow the containing block).
-     
 
 	Note: These rules specify the previously-undefined behavior of this cyclic case in <a href="https://www.w3.org/TR/CSS2/visudet.html#the-width-property">CSS2&sect;10.2</a>.
 	Note also, the behavior in <a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property">CSS2&sect;10.5</a>
 	is superseded in their respective specifications for layout modes
 	(such as <a href="http://www.w3.org/TR/css-flexbox/">flex layout</a>)
 	not described in CSS2.
 
+	Similarly, percentage margins and padding behave as zero in such cyclic cases
+	when calculating the containing block's size,
+	and then resolve when calculating the used sizes and positions of its content.
+	(This defines the previously-undefined behavior of this cyclic case in
+	<a href="https://www.w3.org/TR/CSS2/box.html#margin-properties">CSS2&sect;8.3</a>
+	and
+	<a href="https://www.w3.org/TR/CSS2/box.padding-properties">CSS2&sect;8.4</a>.
 
 	<div class="example">
 		For example, in the following markup: