[peterkelly]

Adding if statements won't make the language Turing complete; there would need to be some sort of iteration or recursion construct for this to be the case.

If the proposed extension was the only change, and the language remained otherwise declarative, then any mutually-dependent if statements could be easily detected by the implementation (e.g. by constructing an expression graph) and either ignored or interpreted as always true or always false. Excel and other spreadsheet apps already include support for if expressions, and they remain purely declarative.

Having said all this, I think that great care should be taken when considering additions like this to CSS, since it could complicate implementations significantly and may lead to poor performance. All of this can be achieved using javascript, so I don't see why you couldn't just write a javascript library instead. jQuery is an example of this approach, which has proved very popular and hasn't required any changes in standards or browsers.