How does WebKit's JITing style system compare? (As far as I'm aware, nobody else has anything like it, though it is still single-threaded as far as I'm aware.)
I would eventually like to look into a CSS JIT. But I would not like to just copy WebKit—I would like to look into treating CSS as a global DFA and compiling accordingly instead of looking at local optimizations only.
As always, the problem with JITs, especially on the Web, that it's hard to beat the interpreter when you take compilation time into account, and most selectors you see in the wild only apply to a small number of nodes.
For context, [here's an IRC discussion about implementing style matching as a single global DFA, and what it's tradeoffs would be](http://logs.glob.uno/?c=mozilla%23servo#c550146).
Just curious, has anyone seen if there are performance improvements in Javascript DOM manipulation (eg adding elements or attributes in JS)?
Users's don't care specifically about style performance, though, but about things like interactivity. We think Servo's style system will improve those things, but don't have numbers for that on hand.
To give a concrete example, it takes Firefox ~1.2 seconds to restyle the single page HTML5 spec with it's current non-parallel style system. Firefox with Stylo can do this in ~300ms. That is close to a full second off of initial page load.