One really interesting feature of CSS is that it's a regular language (you can write a regex that matches all valid CSS files.) This is unusual even for many config formats.
I understand a proper parser would (perhaps) be more complex, but could also be open sourced in a lower level lang such as C, and then used by everyone, there's also already the full test suites for testing the output - maybe even Google+MS+Moz could open a contest - "fastest, most readable and efficient parser in C (or rust, wtv) takes 10k home"