[css-sizing-3] Define term preferred aspect ratio. w3c#4962

Author: fantasai

css-sizing-3/Overview.bs

@@ -293,6 +293,17 @@ Intrinsic Size Constraints</h3>
 		<dd>
 			A sizing constraint imposed by the box's <a>containing block</a>
 			that causes it to produce its <a>min-content contribution</a>.
+
+		<dt><dfn lt="preferred aspect ratio | aspect ratio">preferred aspect ratio</dfn>
+		<dd>
+			A width:height ratio inherent to a box,
+			which biases various sizing algorithms
+			to produce a size consistent with that ratio
+			insofar as possible while honoring other sizing inputs.
+			Unless otherwise specified,
+			a box’s [=preferred aspect ratio=] is its [=intrinsic aspect ratio=] if any
+			and is applied to its [=content box=].
+			Most boxes do not have a [=preferred aspect ratio=].
 	</dl>
 
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