It is functionally identical to Emacs-25.3, except for a fix to the Windows build process. The original 64 bit binaries for 25.3 were not able to run on all target platforms. The 32 bit binaries are unaffected.
The changes are available in commit 7b3d1c6beb54ef6c423a9, and the issues fully described on the emacs-devel mailing list.