[smsm42]

Or they are performant now. But once people start writing CSS code the same way the write JS code, they stop to be. You can still write super-tight code in ASM (or eve C) and it will be blazing fast. Almost nobody does it, because it's too hard. Once people start writing CSS the same way, it'll become slow and bloated too.

[zarzavat]

The big performance sink in CSS is rule matching, or layout if you consider that to be part of CSS.

Efficient evaluation of expressions is a solved problem.

Having conditionals would actually improve performance because you can use fewer rules.