I'm mildly interested in impl details. But I'm more interested in what it speeds up and how it was measured (ideally in a form where I could try my own old-to-new and cross-browser comparisons, but just numbers would be interesting too).
We get significant speed ups from parallelism and such on big doms over all during page load, but we also get speedups from dynamic change handling implementing smarter invalidation than Gecko, which basically restyles the whole subtree / every sibling if it finds a relevant combinator affected by a change.
WebKit does much better than Gecko for class and other attribute changes, at least for descendants, where you go down with the relevant selectors. Stylo's system is relatively similar, but doing selector matching ltr, and handling the same way state and id changes.
You can see components/style/invalidation/element in the servo repo for the relevant code in that regard.