AFAIK Servo is mostly a DOM layout parallel renderer. I mistook it for a full browser but the dev corrected me saying it's "just" rendering engine. Right now they're using separate css lib, js interpreter, etc. Servo deals with the layout.
There is a plan to replace the separate CSS library with its own.
Yeah they were saying it was probably causing performance issues, so the sooner the better.
It is a (long, hard) work in progress. One dev started this huge task about 3 weeks ago :-)
I hope this didn't come across like an impatient rant. I just wish them a quick path toward advancing state of the art ideas about DOM rendering and parallelism.