Probably because they use unexposed, undocumented APIs, but non-hardware accelerated CSS animations that trigger repaints (width, top, etc) in Safari are much, much smoother than in Chrome and FF. They look almost as good as accelerated animations.

Unfortunately I can't really commit to using Safari on desktop because they don't have good ad blocking extensions (please correct me if I'm wrong). Really like FF for this.

But I really appreciate how Apple is committed to improving and highlighting Safari. After totalitarian regimes, browser engine monoculture is the greatest threat to Internet freedom I can think of.