[css-sizing] Floor stretch-fit size at zero.

Author: fantasai

css-sizing/Overview.bs

@@ -125,7 +125,10 @@ Terminology</h2>
 			The <a>stretch fit</a> into a given size
 			is that size,
 			minus the element's computed margins (not collapsed, treating ''margin/auto'' as zero),
-			border, and padding in the given dimension.
+			border, and padding in the given dimension
+			(such that the outer size is a perfect fit),
+			and flooring at zero
+			(so that the inner size is not negative).
 
 			<p class='note'>
 				Note: This is the formula used to calculate the ''width/auto'' widths
@@ -578,7 +581,8 @@ Stretch-fit Sizing</h3>
 	<p>
 		&hellip;less the box's <a>inline-axis</a> margins
 		(after any <a href="https://www.w3.org/TR/CSS21/box.html#collapsing-margins">margin collapsing</a>, and treating ''margin/auto'' margins as zero),
-		borders, and padding.
+		borders, and padding,
+		flooring at zero.
 
 		Issue: Shouldn't this be ignoring margin collapsing?