Earlier in the year, Microsoft updated the Xbox One to enable playback of videos using the MKV container format, widely popular for TV and movie rips. The Xbox Music app for Windows 8.1 has just received an update to likewise enable MKV support in the Metro-style video player.
Cursory testing showed that both MPEG-4 and H.264 video codecs were supported, with AAC, HE-AAC, AC3, and MP3 audio all working. The relative newcomer H.265 and open source friendly Theora and VP8 video codecs did not appear to work.
Windows 10 will go a step or two further than this with its media support. The MKV support in the Xbox Video app is specific to that app; it doesn't enable the rest of the system to use MKV files, with both third-party apps and Microsoft's own Windows Media Player being left out. In Windows 10, Microsoft is including system-level support for both MKV containers and FLAC lossless audio.