Webkit keeps prefixes usually because it is not implemented fully and it is an experimental feature. They began unprefixing flexbox in Sept. 2014, I don't know when it's gonna ship.

Though, its super unfair that the Web Inspector in Webkit uses unprefixed Flexbox (http://trac.webkit.org revision 173707).

Safari is the new IE. No idea why they still require vendor prefix for a range of accepted and implemented standards.

The world is a cruel place :)