Safari Technology Preview is a version of Safari for OS X, distributed by Apple, that includes a cutting-edge, in-development version of the WebKit browser engine. It’s a great way to test upcoming WebKit features and give feedback to the people building them when it’s most useful — early in development.
Think Webkit Nightly, but then in a Safari wrapped featureset (iCloud History, and iCloud Tabs). Features you can try now – which are currently not included in the latest stable Safari release – are ES6, Shadow DOM, CSP2, etc.
Safari Technology Preview →
Introducing Safari Technology Preview →