I remember reading that Chromium's engine is slower sometimes because it follows the standard more strictly. https://codeberg.org/teddit/teddit/issues/24#issuecomment-16...
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.