[css-images-3] Carry over the degenerate ratio handling to images w3c#4572

Author: tabatkins

css-images-3/Overview.bs

@@ -1176,6 +1176,10 @@ Object-Sizing Terminology {#sizing-terms}
 			can have an <a>intrinsic width</a> and an <a>intrinsic height</a>,
 			but no <a>intrinsic aspect ratio</a>.
 
+			If an [=object=] has a [=degenerate ratio|degenerate=] [=intrinsic aspect ratio=]
+			(at least one part being zero or infinity),
+			it is treated as having no [=intrinsic aspect ratio=].
+
 			If an [=object=] (such as an icon) has multiple sizes,
 			then the largest size (by area) is taken as its intrinsic size.
 			If it has multiple aspect ratios at that size,