According to people familiar with the latest OS X Mavericks 10.9.3 beta, dubbed build 13D38, Apple is again requesting developers focus on main graphics drivers, audio enhancements and Safari, three categories that have been ares of concern since the first 10.9.3 beta.
Build 13D38 comes one week after the last version and contains no known issues. Apple specifically points to Graphics Drivers, Audio, Mail, Safari and iTunes contacts and calendar synchronization as areas of interest.
Prior beta releases of OS X 10.9.3 Mavericks incorporated pixel-doubling scaling support, suggesting the update will bring a so-called "Retina" output mode for users connecting their late-2013 MacBook Pro with Retina display to external 4K monitors
Developers can download the build 13D38 from Apple's Developer Portal or via Software Update.