WebM
Google's open container format for web video — a deliberately restricted subset of Matroska, free of licence fees.
The format was announced in 2010, while the fight over which codec HTML5's <video> should use was under way. The point was to offer a wholly royalty-free option: WebM as container, VP8 or VP9 for picture, Vorbis or Opus for sound. AV1 is now often used.
The restriction is deliberate — fewer options means fewer ways to produce a file some player cannot handle. Support exists in every browser, with Safari the last to join. Outside the web the format is uncommon; editing software and televisions still expect MP4.