IE7 invokes a DHTML behavior to load and parse all style sheets into a form that Explorer can understand. You can then use most CSS2 selectors without having to resort to CSS hacks.
The behavior is lightweight and is automatically loaded via a CSS inclusion. No alteration of your original document is necessary. Nor do you have to alter your CSS.
A very clever solution to a very irritating problem. Especially with all the IE rendering bugs
See also: Is Internet Explorer Living on Borrowed Time?