[css-grid] Clamp automatic minimum by fixed track sizes. (Issue #283)

Author: fantasai

css-grid/Overview.bs

@@ -1207,6 +1207,11 @@ Implied Minimum Size of Grid Items</h3>
 	(The effect is analogous to the <a>automatic minimum size</a>
 	imposed on <a>flex items</a> in their <a>main axis</a>.)
 	[[!CSS-FLEXBOX-1]]
+	However, if the <a>grid item</a> spans only <a>grid tracks</a>
+	that have a <a lt="fixed sizing function">fixed</a> <a>maximum sizing function</a>,
+	its <a>automatic minimum size</a> in that dimension
+	is further clamped to less than or equal to the size necessary
+	to fit its margin box within the track (flooring at zero).
 
 	<div class="note" id="min-size-opt">
 		Note that while a content-based minimum size is often appropriate,