In a recent post, I pleaded for help with CSS. Among some of the great suggestions, I stumbled across an interesting feature supported by IE... Conditional Comments. These are evaluated on the client-side. In the context of CSS, it would enable a developer to allow IE to determine whether or not to process a link tag.
Here are some simple examples (not related to CSS):
<!--[if IE 5]><p>You are using Internet Explorer v.5</p><![endif]-->
<!--[if IE 6]><p>You are using Internet Explorer v.6</p><![endif]-->
<![if !IE]><p>Your browser is either old or not IE</p><![endif]>
Observe the lack of <!-- in the last example. This means the browser will "reveal" the inner content if it is either downlevel or not IE.