Apple has previously released “nightly builds” of WebKit, but this seems to be a new program to allow web developers to easily run Safari and WebKit builds for testing alongside the released version of Safari. This should be great for web developers trying to take advantage of new WebKit features. This may also be a sign that Apple is looking at implementing some advanced web technologies that have existed in Chrome but have often been thought were less important for Apple to implement due to the importance of native apps to the Apple platforms.
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.