It's so sad. As a web developer (sometimes), I used to do CSS and like almost all CSS developers you will have some trouble. A bad but fast solution I used to do is to duplicate CSS statement for IE, like this one:

body {
  background-color: green; /*  Green for everybody */
  _background-color: red; /*  Overload to red for Internet Explorer */
}

But this trick is not working anymore with IE7, it doesn't understand the underscore... the solution? Add a point!

body {
  background-color: green; /*  Green for everybody */
  _background-color: red; /*  Overload to red for Internet Explorer 6 */
  .background-color: blue; /*  Overload to blue for Internet Explorer 7 */
}

This is really sad! First of all, the old hack is well none and used... so, lots of CSS are actually not working like it should do with IE7. Why the heck they did that? Isn't Microsoft good are retro-compatibility? Thought so....