[css-sizing-4] aspect-ratio should interpolate by computed value. Fixes w3c#5735

Author: tabatkins

css-sizing-4/Overview.bs

@@ -160,6 +160,7 @@ Preferred Aspect Ratios: the 'aspect-ratio' property</h3>
 	Inherited: no
 	Applies to: all elements except <a>inline boxes</a> and internal ruby or table boxes
 	Computed value: specified keyword or a pair of numbers
+	Animation type: by computed value
 	</pre>
 
 	This property sets a <dfn export>preferred aspect ratio</dfn> for the box,