[css-sizing-3] Switch min-width/height: auto to computing to auto alw…

…ays, resolved value as zero where needed, per dholbert's proposal in #2248. Pending WG approval.

css-sizing-3/Overview.bs

@@ -360,7 +360,14 @@ Sizing Values: the <<length-percentage>>, ''width/auto'' | ''max-width/none'', '
 			For 'min-width'/'min-height',
 			specifies an <dfn dfn for>automatic minimum size</dfn>.
 			Unless otherwise defined by the relevant layout module,
-			this computes to zero.
+			this behaves as zero.
+
+			For backwards-compatibility,
+			the <a>resolved value</a> of this keyword is zero
+			for boxes of all [[!CSS2]] <a>display types</a>:
+			block and inline boxes,
+			inline blocks,
+			and all the table layout boxes.
 
 		<dt><dfn for="max-width, max-height">none</dfn>
 		<dd>