Maybe not Safari, but they have open sourced WebKit and a lot of kernel modules and other useful things: https://developer.apple.com/opensource/
https://news.ycombinator.com/item?id=25675763
I don't know how much of it is lip-service, but webkit.org talks about using webkit on non-Apple platforms, and webkit-gtk gets discussed on their mailing list.
It feels to me like a truly collaborative open-source project, in a way that's unlike most of what Apple does. Again, I don't know how much of this is an aberration, but I've always been impressed with the whole thing.