Apple seeded a new build of OS X 10.9.3 to developers on Tuesday. Build 13D43 was seeded just under a week after Apple seeded build 13D38.
Apple is still asking that developers focus their efforts on testing Graphics Drivers, Audio, Mail, Contacts and Calendar sync over USB in iTunes, and Safari. 10.9.3 also adds support for 4K displays, which offers improved readability and 60Hz output from a Retina display MacBook Pro.
The beta is available via the Update tab in the Mac App Store and also via the Mac Dev Center.
