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.
The definition is a little fluid. At the absolute minimum you'd need an HTML renderer and Internet access, but this wouldn't get you so far on the modern Web. Most people would expect some level of CSS and JavaScript support too.