ffmpeg encoded videos are not playing on mobile browser
Unfortunately due to patent/licensing issues1, MP4 playback is not supported natively in all browsers. That's where file format redundancy comes into play; if one file format is not able to be natively loaded by a system a backup version may be able to work.

I have found that certain browsers will be able to run files encoded with the .webm container using VP8 plus Vorbis for audio and video, respectively, out of the box while using HTML5 players.2

Citations



1 https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats#Browser_compatibility

2 https://en.wikipedia.org/wiki/HTML5_video#Multiple_sources