[css-sizing] Define case of images with intrinsic aspect ratio but no…

… size whose containing block depends on their size (undefined in CSS2.1). Fixes #951.

css-sizing-3/Overview.bs

@@ -395,8 +395,9 @@ Intrinsic Sizes</h3>
 				use the <a>stretch fit</a> into that size for the inline size
 				and calculate the block size using the aspect ratio.
 			* Otherwise
-				use a width of <span class="issue">UNDEFINED</span>
-				and calculate the height using the aspect ratio.
+				use a width of ''300px''
+				(height of ''150px'' in <a>vertical writing modes</a>)
+				and calculate the other dimension using the aspect ratio.
 		<dt>For boxes without an intrinsic aspect ratio:
 		<dd>
 			* If the <a>available space</a> is <a>definite</a>