[css-sizing] Clarify how %s work when resolving against an intrinsic containing block, with example.

Author: tabatkins

css-sizing/Overview.bs

@@ -569,9 +569,31 @@ Fill-available Sizing</h3>
 		Maybe it should stop at each formatting root, or something similar?
 
 <h3 id="percentage-sizing">
-	Percentage Sizing</h3>
+Percentage Sizing</h3>
 
 	Percentages specify sizing of a box with respect to the box’s <a>containing block</a>.
+	When calculating the <a>intrinsic size contribution</a> of the box,
+	intrinsic sizes are instead treated as ''width/auto''.
+
+	<div class="example">
+		For example, in the following markup:
+
+		<pre class='lang-html'>
+			&lt;div style="width: min-content">
+				&lt;div style="width: 50%;">
+					LOOOOOOOOOOOOOOOOOOOONG
+				&lt;/div>
+			&lt;/div>
+		</pre>
+
+		When calculating the width of the outer div,
+		the inner div is treated as ''width: auto'',
+		so the outer div sets itself to the width of the long word.
+		Since the outer div's width didn't depend on "real" layout,
+		though, it's treated as <a>definite</a>
+		for resolving the inner div,
+		whose width resolves to half that of the outer div.
+	</div>
 
 	Although this may require an additional layout pass to re-resolve percentages in some cases,
 	the ''min-width/auto'', ''min-width/min-content'', ''min-width/max-content'', and ''min-width/fit-content'' values