Doing everything in parallel is pretty much the point of the project. I'm not directly involved but I have been watching the project. There a few specific parts of layout that can't be parallelized[0] but pretty much every page [1][2] has work that can be done in parallel. The last batch of perf numbers I know of is from last November [3] and I'm unaware of any major perf regressions.
In benchmarks from last fall, Servo was twice as fast as modern engines for layout tasks even in single-threaded mode (and four to five times faster when using four cores). Who knows, this may regress as they implement more of the web platform, but Rust and Servo are also getting faster all the time. It will be fascinating to watch it progress over the next few years.
Unless you have CSS floats affecting the whole page, 90% of DOM elements in one parent element or other pathological cases my impression was that it is.
Small nit: The fact that most elements on the page are immediate children of one parent doesn't necessarily negatively affect parallelism. It only will if all those children are "display: inline". (For example, if your entire page is one giant paragraph hundreds of thousands of words long, then you won't get much parallelism out of layout. But you will if you have hundreds of thousands of separate paragraph elements, even if they're all siblings of one another.)
Is this clear? The multi-core rendering surely can't be used for every page.