My understanding from reading about this on the CSS working group boards is as follow - the biggest issue is users assume CSS is instantaneous. If you ever see

My understanding from reading about this on the CSS working group boards is as follow - the biggest issue is users assume CSS is instantaneous. If you ever see an interaction (other than animations), or a flash of unstyled content, it looks like a broken website. That means the W3C want to ensure any candidate they are using can be calculated on poor performance machines in a matter of a couple milliseconds. For one off variables this is not hard at all, but the the cascading effect of css means that a lot more calculations have to be made