[css-sizing-4] Define effect of aspect-ratio on margin collapsing. w3c#5328

Author: fantasai

css-sizing-4/Overview.bs

@@ -216,6 +216,10 @@ 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.
 
+	For the purpose of margin collapsing ([[css2/box#collapsing-margins]]),
+	if the [=block axis=] is the [=ratio-dependent axis=],
+	it is not considered to have a [=computed value|computed=] 'block-size' of ''height/auto''.
+
 	If a replaced element's only [=intrinsic dimension=]
 	is an intrinsic width or an intrinsic height,
 	giving it a [=preferred aspect ratio=]