With Windows 10, Media Player supports the MKV container out of the box, as confirmed by Gabriel Aul, providing updates on the Windows Insider program on Twitter.
When asked later by another user whether Microsoft will include a codec for H.265/HEVC video streams, Aul confirmed that HEVC is "also supported in-box."
The promise of HEVC is to make it possible to stream and download ultra high resolution video content without having to push up the bitrate to the same degree necessary with other standards.
Windows 10 is set to ship in late in 2015.