That's not a problem at all. If it's worth it to do it, people will do it even if it takes 200 lines. If it's not, they won't bother. So, even with it being achievable as a "first-class" option in CSS4, I don't see any particular rise of this exploit happening.

It's like arguing that something is safer because it requires 10 lines of code instead of one to do the exploit. That's security by obscurity.