Comment # 6 on bug 954646 from Raymond Wooninck

Maybe you should really install the chromium-ffmpeg package and then do the
same test.

The link https://www.youtube.com/watch?v=0NTOZbjg6SE  shows the Google Nexus 5X
Review!.  Clicking on stats for nerds I see as in attached file
video1_stats.png (indicating video/webm; codecs="vp9"). 

The link https://www.youtube.com/watch?v=XAoQPp9XLt8 shows the iPad Pro Second
Impressions! [Apple Pencil] from the same author. With the ffmpeg package this
file plays fine and clicking on stats for nerds I see as in attached file
video2_stats.png (indicating video/mp4; codecs="avc1.4d401fp").  

So the second video is definitely in another format than webm/vp9 and mp4 is
clearly a codec that is only supported from the ffmpeg package.