I am experimenting with *.webm videos. I've created several *.webm videos using different bit rates and in all cases the playback quality in MythTV FE is poor - highly pixelated. However, when viewing the same videos using VLC player, these videos appear much better (some slight/occasional digital artifacts are apparent but not enough to ruin the viewing experience).

I'll admit that my settings to create *.webm are not yet optimized using avconv, but again, VLC plays the videos I've created just fine.

Why does the playback quality of *.webm depend on the player?
Why are there noticeable differences among players?
Why is MythTV's player so different?