[css-sizing-4] Add syntax to specify ratio as fallback, deferring to replaced intrinsic ratio if available. #4951

Author: fantasai

css-sizing-4/Overview.bs

@@ -178,7 +178,7 @@ Intrinsic Aspect Ratios: the 'aspect-ratio' property</h3>
 
 	<pre class="propdef">
 	Name: aspect-ratio
-	Value: auto | <<ratio>>
+	Value: auto || <<ratio>>
 	Initial: auto
 	Inherited: no
 	Applies to: all elements except <a>inline boxes</a> and internal ruby or table boxes
@@ -216,6 +216,15 @@ Intrinsic Aspect Ratios: the 'aspect-ratio' property</h3>
 			Size calculations involving <a>preferred aspect ratio</a>
 			work with the dimensions of the box specified by 'box-sizing'.
 
+		<dt><dfn>auto && <<ratio>></dfn>
+		<dd>
+			If both ''auto'' and a <<ratio>> are specified together,
+			the [=preferred aspect ratio=] is the specified ratio
+			of ''<var>width</var> / <var>height</var>''
+			unless it is a [=replaced element=] with an [=intrinsic aspect ratio=],
+			in which case that aspect ratio is used instead.
+			In all cases, size calculations involving this aspect ratio
+			work with the [=content box=] dimensions always.
 	</dl>
 
 	Note: If a box has both a 'width' and a 'height'