The Safari Technology Preview is described by Apple this way:
"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."What's cool is that Safari Technology Preview is it's own app and can run in parallel with the "shipping" version of the Safari web browser on your Mac. Safari Technology Preview is also signed by Apple, giving it the capability of participating in Apple's web services like iCloud Drive, and bookmarks syncing. Safari Technology Preview also gives web developers new powerful tools and tech nerds a look at the technologies that will be coming to Safari and WebKit in the future.
For those who are interested in taking Preview out of a test drive, you can download it for free from the Apple Developer Website. Updates for Safari Technology Preview will be released every few weeks from the Mac App Store.