[selectors] Add example showing the difference between general case-insensitivity and strict ASCII case insensitivity, #13645

Author: svgeesus

selectors-4/Overview.bs

@@ -2151,6 +2151,13 @@ Case-sensitivity</h3>
 	[=ASCII case-insensitively=]
 	(i.e. [a-z] and \[A-Z] are considered equivalent).
 
+	<div class="example">
+		For example, ASCII case insensitivity allows <code>green</code>
+		to match <code>GREEN</code>.
+		However, <code>grün</code> 
+		would not match <code>GRÜN</code>.
+	</div>
+
 	Alternately, the attribute selector may include the identifier <code>s</code>
 	before the closing bracket (<code>]</code>);
 	in this case the UA must match the value case-sensitively,