"WebKit is an open source web browser engine. WebKit is also the name of the Mac OS X system framework version of the engine that's used by Safari, Dashboard, Mail, and many other OS X applications." Annotated link http://www.diigo.com/bookmark/http%3A%2F%2Fwebkit.org
NightShift automatically downloads and updates WebKit, the Safari HTML rendering engine, to the latest nightly version. No user intervention is required, everything is fully automated.