Safari Technology Preview →

Apple:

Safari is the best way to see the sites on iPhone, iPad, and Mac. Safari Technology Preview gives you an early look at upcoming web technologies in OS X and iOS including the latest layout technologies, visual effects, and developer tools so you can provide input on how they are implemented and deliver a best-in-class user experience on all Apple devices.

As Jason Snell points out, Apple previously released “nightly builds” of WebKit, but this new program looks to offer a more structured testing platform.