[css-sizing-4] More clarifying edits, and an added example, for the 'try your best to fill your container' text for width/height:stretch. w3c#1614

Author: tabatkins

css-sizing-4/Overview.bs

@@ -654,9 +654,11 @@ Stretch-fit Sizing: filling the containing block</h3>
 	Additionally,
 	in [=formatting contexts=] and axes in which the relevant [=self-alignment property=] does not apply
 	(such as the block axis in Block Layout, or the main axis in Flex Layout),
-	[=stretch-fit sizing=] still has the effect
-	of causing the box to attempt to fill its containing block,
-	invoking the percentage size calculations but applying that size to its margin box
+	in cases where a percentage size in that axis would resolve to a definite value,
+	a [=stretch-fit size=]
+	causes the box to attempt to fill its containing block--
+	behaving as ''100%''
+	but applying the resulting size to its margin box
 	instead of the box indicated by 'box-sizing'.
 	For this purpose, ''margin/auto'' margins are treated as zero,
 	and furthermore, for [=block-level boxes=] in particular,
@@ -665,6 +667,10 @@ Stretch-fit Sizing: filling the containing block</h3>
 	if its parent’s [=sizing properties=] all had their [=initial values=],
 	then its block-start/block-end [=margin=] is treated as zero.
 
+	Note: Consequently, if neither ''align-self/stretch'' alignment applies
+	nor percentage sizing can resolve,
+	then the box will resolve to its [=automatic size=].
+
 	<div class="example">
 		For example, given the following HTML representing two [=block boxes=]:
 		<pre class=html>
@@ -699,6 +705,19 @@ Stretch-fit Sizing: filling the containing block</h3>
 		</pre>
 	</div>
 
+	<div class="example">
+		On the other hand,
+		in this example the container's height is indefinite,
+		causing a percentage height on the child to [=behave as auto=],
+		so ''height: stretch'' [=behaves as auto=] as well.
+
+		<pre>
+		  .outer { height: auto; margin: 0; }
+		  .inner { height: stretch; margin: 10px; }
+		</pre>
+	</div>
+
+
 <h3 id="contain-fit-sizing">
 Contain-fit Sizing: stretching while maintaining an aspect ratio</h3>