Hidde de Vries gathers some of the early thinking about CSS:
There is quite a bit of information on the web about how CSS was designed. Keeping it simple was a core principle. It continued to be — from the early days and the first implementations in the late nineties until current developments now.
The four main design principles listed are fascinating:
Authors can specify as much or little as they want
It is not a programming language by design
Did CSS get more complicated since the late nineties? is a post from CSS-Tricks