[css-sizing-4] Use 'box-sizing' box for explicit aspect ratios, content-box for intrinsic ones. w3c#4007

Author: fantasai

css-sizing-4/Overview.bs

@@ -98,11 +98,16 @@ Intrinsic Aspect Ratios: the 'aspect-ratio' property</h3>
 			<a>Replaced elements</a> with an <a>intrinsic aspect ratio</a>
 			use that aspect ratio;
 			otherwise the box has no <a>preferred aspect ratio</a>.
+			Size calculations involving <a>intrinsic aspect ratio</a>
+			work with the <a>content box</a> dimensions always.
 
 		<dt><dfn><<ratio>></dfn>
 		<dd>
 			The box’s <a>preferred aspect ratio</a> is the specified ratio
 			of ''<var>width</var> / <var>height</var>''.
+			Size calculations involving <a>preferred aspect ratio</a>
+			work with the dimensions of the box specified by 'box-sizing'.
+
 	</dl>
 
 	Note: If a box has both a 'width' and a 'height'