Ricky Mondello, from Apple’s Safari team:
Starting today, there’s a new, convenient way to see what features and improvements are coming to Safari and other applications that use WebKit. 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.
Installing this doesn’t affect your existing copy of Safari and they can be run side by side to compare behaviors between them. I’ll be installing this on my MacBook later tonight. Though, given how infrequently I use the machine, I don’t expect I’ll launch it very often.