MKV
Matroska — the open container format that can hold essentially anything, in any number of tracks.
The format started in 2002 with the aim of being unbounded: any number of audio tracks, subtitle tracks and chapter markers, any codecs, embedded fonts for properly styled subtitles. The name comes from the Russian nesting dolls, describing its hierarchical structure.
That flexibility made it standard in enthusiast circles and the anime scene, where multiple audio tracks and subtitles are the norm. The downside is weaker support in phones and televisions, which often only handle MP4. WebM is a deliberately restricted subset of Matroska, intended for the web.