[css-sizing-4] Add stretch + fit-content + contain keywords.

See https://lists.w3.org/Archives/Public/www-style/2017Feb/0061.html and https://lists.w3.org/Archives/Public/www-style/2017Feb/0059.html and https://lists.w3.org/Archives/Public/www-style/2017Nov/0001.html

Author: fantasai

css-sizing-4/Overview.bs

@@ -97,8 +97,33 @@ Sizing Properties</h3>
 ISSUE(820): Add shorthands.
 
 <h3 id="sizing-values" oldids='width-height-keywords'>
-Sizing Values: the <<length-percentage>>, ''width/auto'' | ''max-width/none'', ''width/min-content'', ''width/max-content'', and ''width/fit-content()'' values</h3>
+New Sizing Values: the ''stretch'', ''fit-content'', and ''contain'' keywords</h3>
 
+	<pre class="propdef partial">
+	Name: width, height, inline-size, block-size, min-width, min-height, min-inline-size, min-block-size, max-width, max-height, max-inline-size, max-block-size
+	Values: stretch | fit-content | contain
+	</pre>
+
+	<dl dfn-type=value dfn-for="width, height, inline-size, block-size, min-width, min-height, min-inline-size, min-block-size, max-width, max-height, max-inline-size, max-block-size">
+		<dt><dfn>stretch</dfn>
+		<dd>
+			Applies [=stretch-fit sizing=],
+			attempting to match the size of the box’s [=margin box=] to the size of its [=containing block=].
+
+		<dt><dfn>fit-content</dfn>
+		<dd>
+			Essentially ''fit-content(stretch)''
+			i.e. min(''max-content'', max(''min-content'', ''stretch'')).
+
+		<dt><dfn>contain</dfn>
+			If the box has a [=preferred aspect ratio=],
+			applies [=contain-fit sizing=],
+			attempting to fit into the box’s constraints
+			while maintaining its [=preferred aspect ratio=] insofar as possible.
+
+			If the box has no [=preferred aspect ratio=],
+			applies [=stretch-fit sizing=].
+	</dl>
 
 <h2 id="ratios">
 Aspect Ratios</h2>