Invalid WebM files

[Nes-si]

Hi there.
For default the lib makes invalid WebMs. WebM format implies only VP8 or VP9 video codecs (WebM Project). But for default the lib creates WebM with AVC codec. It's a nonstandard file that causes multiple errors. For me, Firefox on Mac can't play the video. Chrome can, but the timeline isn't usable (the problem describes here: #581 and #561 )

[uffou]

I am also looking for a way to fix the timeline as vp9 seems like the most efficient format.