Like Windows 10 Desktop -- which runs on PCs, laptops and tablets -- Spartan is still not feature-complete at this point. The first Spartan preview does, however, include most of the functionality that Microsoft execs showed off on January 21 during an early demo of Spartan as part of a Windows 10 press event.
The new Reading View, which allows users to view content with fewer distractions and/or save it for later, also is in Spartan in today's new test build. (The ability to read saved content offline is not yet enabled in today's test build.)
The Spartan browser, which will be pinned to the Windows 10 Desktop task bar, includes only the new "Edge" rendering engine. At the same time, IE 11, which also is bundled with the Windows 10 Desktop, includes only the "Trident" (MSHTML) rendering engine -- not both the Edge and Trident ones -- and is there for backwards compatibility. IE 11 is not pinned to the task bar in Windows 10 Desktop, but it is still meant to be readily discoverable and usable (and pinnable, if users want to do so).
There are a few bug fixes in today's new Windows 10 Desktop build but no other noteworthy new features beyond Spartan.
Windows 10 Mobile is designed to run on ARM-based Windows Phones, as well as ARM- and Intel-based small tablets. Windows 10 Mobile is expected to include Spartan, but not IE 11, when the first browser-inclusive version of it is available to testers, as well as when it's commercially available.