Requested feature overview description.
Right now watching WebM file on software that doesn't support it gives following error:
If you are using Internet Explorer 9 or higher, download this software to view WebM videos.

However this is only applicable with Internet Explorer, which microsoft is also already phasing out in favor of Edge. This solution sadly also doesn't seem to work with VP9/Opus using posts.

For edge this solution doesn't work, user needs to download Web Media Extensions from Microsoft Store instead. (thanks forum #248379)
https://www.microsoft.com/store/productId/9N5TDP8VCMHS

There are also cases where user cannot do anything to their browser, so instructing user to download the file and trying to view it with media player would also be helpful as many users might not realize webm files being regular video files. Something like VLC player would be easiest to recommend (even if it's nowhere near perfect) as it also applies with iOS.

Why would it be useful?
As we already have the system in place and doesn't show up regularly, so makes sense to use it for all possible scenarios users might run into.

What part(s) of the site page(s) are affected?
WebM posts when video cannot be viewed.

Might be good to set up a wiki for it and link to that instead. That way you can #header segments for users to immediately jump to detailed instructions.