[css-sizing-3] Add another level of indirection to the example, to hopefully make #4177 clearer.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -971,12 +971,17 @@ Extrinsic Size Determination</h2>
 
 		<xmp class='lang-html'>
 			<article style="height: 60em">
-				<aside style="height: 50%;">
-				</aside>
+				<figure style="height: 50%;">
+					<img style="height: 50%;">
+				</figure>
 			</article>
 		</xmp>
 
-		the <code>&lt;aside></code> would be 30em tall.
+		* the <code>&lt;figure></code> would be ''30em'' tall
+			= 50% of the [=definite=] ''60em'' height of the <code>&lt;article></code>
+		* the <code>&lt;img></code> would be ''15em'' tall
+			= 50% of the <code>&lt;figure></code>’s height
+			(which is itself [=definite=] because it's a percentage resolved against a [=definite=] length)
 	</div>
 
 	Sometimes the size of a percentage-sized box’s <a>containing block</a>