[css-conditional-4][editorial] Add an example

Author: svgeesus

css-conditional-4/Overview.bs

@@ -63,6 +63,21 @@ Extensions to the ''@supports'' rule</h2>
 	js/CSS-supports-L4.html
 </wpt>
 
+<div class="example" id="ex-selector">
+	This example tests whether the
+	<a>column combinator</a>  (||)
+	is supported in selectors,
+	and if so uses it to style particular cells in a table.
+
+	<pre class="lang-css">
+		@supports selector(col || td) {
+			col.selected || td {
+				background: tan;
+			}
+		}
+	</pre>
+</div>
+
 <h3 id="support-definition-ext">
 Extensions to the definition of support</h3>