From a blog:
The basic idea with CSS expressions is that you will have calculation and dynamic values for properties in the CSS code, something
Expressions are unique to IE and were dropped in 8:
http://msdn.microsoft.com/en-us/library/cc304082(VS.85).aspx#expressions
More about expressions:
http://msdn.microsoft.com/en-us/library/ms537634(v=VS.85).aspx