Hot take... There is something about programming projects spending 10x (or 1000x) the effort to parallelize execution when everything is still restricted by von Neumann architecture.
Just that - even if you distribute work to a number of cores (that itself limited), you still need to sync the execution. So unless you have some bigger chunks of computation, the performance increase will be very small.
And yes, there are bigger chunks of work when rendering a web page, but that is typically already outside the css engine and can be distributed without having to rewrite the core engine.
But my comments are a bit generic here, so I am open to comments regarding this particular project.