Fwiw, I've seen a number of cases of the other way around: the WebKit/Chrome CSS implementation had some shortcuts for performance reasons that caused them to not follow the spec, where Firefox did.

The link you posted just indicates that Firefox rendered the page faster than Chrome; did it render it _wrong_?

One other note: the linked issue here is layout, not the CSS system per se; this part is not parallelized in Gecko and is still in C++, not in Rust.