div {
  background-color: #999; /* all browsers */
  *background-color: #ccc; /* add a * before the property - IE7 and below */
  _background-color: #000; /* add a _ before the property - IE6 and below */
}

http://sixrevisions.com/css/10-random-css-tricks-you-might-want-to-know-about/

The code below will change the background-color of divs depending on what browser the user is viewing the web page under. Since * cascades down to IE7 and below, we use _ after that declaration so that IE6 (and below) has a different background color from IE7.

Target IE6 and IE7 Browsers without Conditional Comments

css, browser, ie6

CSS