WebKit, like FireFox and other open source apps, is updated on a nightly basis, and for those who want to have the very latest features, and don't mind putting up with the occasional browser hang, there are nightly builds available.
That's the background behind my most recent contribution to The Apple Blog, titled Surf on the Edge With a WebKit Nightly Build. Per agreement with them, I will not be cross-posting the piece, but instead, have provided a link. Enjoy.