[css-contain] Highlight HTML code in example (#2615)

Author: Loirooriol

css-contain-1/Overview.bs

@@ -124,7 +124,7 @@ Strong Containment: the 'contain' property</h2>
 
 		For example, assume a micropost social network had markup something like this:
 
-		<pre class='lang-css'>
+		<pre><code highlight=markup>
 			&lt;body>
 				&lt;aside>...&lt;/aside>
 				&lt;section>
@@ -141,7 +141,7 @@ Strong Containment: the 'contain' property</h2>
 					…
 				&lt;/section>
 			&lt;/body>
-		</pre>
+		</code></pre>
 
 		There are probably a <em>lot</em> of messages displayed on the site,
 		but each is independent and won't affect anything else on the site.