On the ‘Insider’ build 14316 of Windows 10, WebM VP9+Opus video files can be played in Microsoft Edge! Well, indirectly via Media Source Extensions, but still. :)
Try it out! https://brionv.com/misc/msetest/ (primitive demo)
You may have to manually enable VP9 in about:flags however, as it defaults to on only if hardware-accelerated, and I have no idea which GPU drivers support that on Windows yet.
It’d be great if HTMLMediaElement and MediaSource’s methods for checking playback support could also allow checking the hw-acceleration status — especially on mobile, it’s often preferable to use a codec that’s hardware-accelerated (like H.264) to use less battery life even if you would get better visual quality from a more advanced codec (like VP9).