If this paragraph is green and has a border (for you colorblind folks), the non-IE6 stylesheet loaded.
If this paragraph is red and italic, the IE6-only stylesheet loaded.
The following method was used:
<!--[if ! lte IE 6]><!--><link rel="stylesheet" href="green.css" media="screen, projection"><!--><![endif]--> <!--[if lte IE 6]><link rel="stylesheet" href="red.css" media="screen, projection"><![endif]-->
Note that the only advantage over the other method is that in theory this uses only "proper" comments, but guess what, the HTML validator still complains.
This works as a technique to include CSS, JS, and other content.