[css-sizing] Per WG resolution, use min size of replaced element, when available, to calculate intrinsic size, rather than defaulting immediately to 300x150. Fixes #1722.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -447,6 +447,17 @@ Intrinsic Sizes</h3>
 				in the <a>inline axis</a>,
 				use the <a>stretch fit</a> into that size for the inline size
 				and calculate the block size using the aspect ratio.
+			* Otherwise
+				if the box has a <a>definite</a> non-zero 'min-width' or 'min-height',
+				use that size and calculate the other dimension using the aspect ratio;
+				if both dimensions have a definite minimum,
+				choose the one that results in the larger overall size.
+
+				Note: This case was previous calculated from a 300x150 default size,
+				rather than the box's min size.
+				This is believed to be a better behavior,
+				and likely to be web compatible,
+				but please send feedback to the CSSWG if there are any problems.
 			* Otherwise
 				use a width of ''300px''
 				(height of ''150px'' in <a>vertical writing modes</a>)
@@ -456,6 +467,9 @@ Intrinsic Sizes</h3>
 			* If the <a>available space</a> is <a>definite</a>
 				in the appropriate dimension,
 				use the <a>stretch fit</a> into that size in that dimension.
+			* Otherwise,
+				if the box has a <a>definite</a> non-zero minimum size ('min-width'/'min-height') in that dimension,
+				use that size.
 			* Otherwise,
 				use ''300px'' for the width
 				and/or ''150px'' for the height