Video player uses MediaElement.js built into WordPress to enable use of HTML5 Video tag when browser supports it, or to fall back to Flash/Silverlight for older browsers.
Support for different formats changes with different browser versions. So, this list is for latest browsers in use currently. Some older browsers will fall back for some file types.
Supports: MP4,WebM. It is expected for Google to remove MP4 support from Chrome.
MS Internet Explorer
Supports HTML5 Video tag since 9.0 and plays: MP4, WebM, WMV.
Supports: MP4, WebM.
New versions since 15 support everything Chrome supports.
In most cases, support for different video formats depend on the device, and even the same OS doesn’t mean that same video files will work. All mobile devices using Android 2.3 and newer or iOS support HTML5 Video and play most of the formats. Android devices can support different formats for Video tag. iOS and WP7/8 devices support MP4.
What format to use?
To attempt using direct HTML5 Video without fallback, best format to use is MP4 or WebM, it works almost everywhere. For old browsers, fall back will be used in any case.