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).
On the other hand in Wikipedia’s case, we don’t use H.264 for patent license issues, so our alternative to software VP9 decoding is JavaScript Theora or VP8 decoding, which is going to be harder on the CPU than nicely tuned native code for VP9 decoding.