[css-sizing] Add a 'behaves as auto' term to Sizing to unify computed/behaves behavior for width/height. Part of w3c#1051.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -302,11 +302,18 @@ New Keywords for 'width' and 'height'</h3>
 	this is true automatically.
 	For Block layout, this means using ''display: flow-root;''.
 
-	<p class='note'>
-		Note that percentages resolved against the intrinsic sizes
-		(''width/max-content'', ''width/min-content'', ''width/fit-content'')
-		will compute to ''width/auto'',
-		<a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property">as defined by CSS 2.1</a>. [[!CSS2]]
+	Note: Percentages resolved against the intrinsic sizes
+	(''width/max-content'', ''width/min-content'', ''width/fit-content'')
+	will behave as ''width/auto''.
+	<!-- <a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property">as defined by CSS 2</a>. [[!CSS2]] -->
+	<!-- Restore the above when 2.1 is updated and published with the new "behaves as" text we resolved on. -->
+
+	To have a common term for both when
+	'width'/'height' computes to ''width/auto''
+	and when it is defined to behave as ''width/auto''
+	(as in the case above),
+	the property is said to <dfn export lt="behave as auto|behaving as auto">behave as auto</dfn>
+	in both of these cases.
 
 <h3 id='the-contain-floats-value'>
 Containing Floats</h3>