(I'm an engineer on Blink's layout engine).
We've recently finished re-architecting Blink's layout engine, part of the reason why we did this investment was we were concerned that we couldn't fix WebKit era bugs (e.g. too many sites would depend on them due to our shared heritage). This makes other engines jobs (e.g. Gecko) super difficult as they'd need to encode even more quirks than they have time for.
I think we've broadly mitigated a large part of that risk. There are still large parts of CSS which are underspecified, e.g. tables/block/float layout. But its slowly getting better.