STR:

1. Visit https://bugs.chromium.org/p/chromium/issues/detail?id=352912#c36
2. Try to play the screencast in that chromium bug-comment (labeled Issue_352912.webm 10.6 MB)

ACTUAL RESULTS:
Unable to play. Firefox says "No video with the supported format and MIME type found

EXPECTED RESULTS:
Video should be playable.

Chrome gives EXPECTED RESULTS. The local video-player tools mplayer and vlc are also able to play the video properly.

(Safari gives similar results to Firefox, though, for what it's worth. So maybe this video is corrupt somehow, or is using some arcane encoding? (It's naming suggests it's webm, but I do see some h264 logging when I view it locally, so I think maybe it's not-really-webm.)

Direct link to video (which also reproduces the issue, since it loads the browser's standalone media viewer UI):
https://bugs.chromium.org/p/chromium/issues/attachment?aid=561686&signed_aid=bXqag_e2-Coc4gZQm-3wDg==&inline=1

I see this in my web console when I load the video in Firefox Nightly:

Media resource https://bugs.chromium.org/p/chromium/issues/attachment?aid=561686&signed_aid=bXqag_e2-Coc4gZQm-3wDg==&inline=1 could not be decoded, error: Error Code: NS_ERROR_DOM_MEDIA_METADATA_ERR (0x806e0006)

The severity field is not set for this bug.
:jimm, could you have a look please?

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This is H264 in matroska, which is legal, but with a .webm extension, which is weird. We don't play regular Matroska file (only Matroska file that conform to the WebM spec). I think we're not going to fix this. Chrome historically doesn't care.

I'm duping against the main bug about supporting Matroska.