It'll probably end up slow and buggy. And likely explode when used with other layouting mechanisms. And it won't benefit from the quite awesome CSS debugging tools we have nowadays.

But yes, more powerful "client side" layouting functionality would be very welcome so we wouldn't have to wait the commitees for a decade to center an element etc. And there is the "Houndini" project to get this going, but the browser support is still extremely poor: https://houdini.glitch.me/