This is already bearing fruit: current Windows Insider builds of MS Edge include support for VP9 video in WebM container via MSE playback (Media Source Extensions).

I've confirmed this works with one of our random VP9 uploads:

• https://brionv.com/misc/msetest/
• https://www.youtube.com/watch?v=nEYrh2SKTwU&feature=youtu.be (screen cap)

Once Opus audio support is added as well (and/or I figure out a way to separately decode the audio via ogv.js and keep them still in sync!) we can look into native WebM VP9 playback on MS Edge, and it'll add some more urgency to getting VP9 transcodes done. Awesome!

Tracked on: https://phabricator.wikimedia.org/T114316