In Servo we've been able to get a lot of parallelism out of the basic building blocks of CSS. Any replacement would have to have a good story here. "Just write your layout in JS" is not good enough and would be a large performance regression.
Constraints based system is the way to go. And yes, it should be native replacement for CSS- a DOM property called constraints instead of style. Unfortunately, JS has to be used for now. [1] As for parallelism, we still have web workers for the solving of constraints computations.
> As for parallelism, we still have web workers for the solving of constraints computations.
I am confident that the lack of shared memory, along with the heavyweight nature of web workers, means that any attempt to use web workers to perform parallel layout will be slower than sequential.
