Can I only use one mp4 video format to play in all browsers?
Yes. You only need to provide one mp4 format to play in all web browsers and devices.
In iPhone, iPad, Android, Chrome, Safari, IE 10 and above, the gallery will use HTML5 to play the mp4 video.
In the legacy web browsers, for example, Internet Explorer 8 and 9, the gallery will use Flash to play the mp4 video.
Firefox and Opera support HTML5, but they do not support mp4. For these two browsers, if you have provided a WebM video, it will use HTML5 to play the WebM. Otherwise, it will use Flash to play the mp4 video.
If your mp4 video does not play in the web browser and devices, one possible reason is your video is not HTML5 compatible. You can use freeware HandBrake http://handbrake.fr/ or FFMPEG https://www.ffmpeg.org/ to re-encode the video.