Ideally it shouldn’t be necessary to provide different CSS for different browsers, but sometimes needs must, particularly when it comes to dealing with the quirks of older versions of Internet Explorer (i.e. IE6 and IE7).
Thankfully there’s a couple of simple hacks that can be used to target CSS at old versions of IE only – the underscore (_) hack for IE6 (and below) and the star (*) hack for IE7 (and below). Just put _ or * in front of a CSS property as required, e.g.:
color: red; /* all browsers */ *color: blue; /* IE7 and below */ _color: green; /* IE6 and below */
Using these rules, this sentence should appear green in IE6 and below, blue in IE7 and red in all other browsers.
There are other hacks that can be used. Check out these blog posts for more:
CSS Tip: Targeting IE 5.x, 6 and 7 Separately