Following the release of the third OS X 10.10.2 pre-release version, Apple has seeded individual Safari betas for testing via its developer program.
While the pre-release version of OS X 10.10.2 included Safari 8.0.3 beta, Apple has made beta versions for OS X Mavericks and OS X Mountain Lion available to download from the developer portal. The latest Safari beta versions include Safari 6.2.3, Safari 7.1.3, and Safari 8.0.3.
Apple lists the following focus areas for the latest beta versions of its web browser:
9to5Mac Happy Hour
General website compatibility
HTML5 video playback on popular video sites
Using the Web Inspector to debug websites
Reading List read/unread state
Editing bookmarks in the Favorites Bar and Bookmarks Sidebar