[css-sizing-4] Specify that a-r can change a replaced element with a single intrinsic dimension into a full suite of 3. #5306

Author: tabatkins

css-sizing-4/Overview.bs

@@ -216,6 +216,14 @@ Intrinsic Aspect Ratios: the 'aspect-ratio' property</h3>
 	at least one of these sizes must be ''width/auto'' (/''flex-basis/content'')
 	for the <a>preferred aspect ratio</a> to have an impact on sizing.
 
+	If a replaced element's only [=intrinsic dimension=]
+	is an intrinsic width or an intrinsic height,
+	giving it a [=preferred aspect ratio=]
+	also gives it an intrinsic height or width,
+	whichever was missing,
+	by transferring the existing size
+	through the [=preferred aspect ratio=].
+
 	Additionally, sizing constraints in either axis
 	(the <var>origin</var> axis)
 	are transferred through the [=preferred aspect ratio=]