Pedantry: WebM is not a compression format. It is a container format. At the moment it appears to support VP8 video, VP9 video, Vorbis audio, and Opus audio;

Pedantry: WebM is not a compression format. It is a container format. At the moment it appears to support VP8 video, VP9 video, Vorbis audio, and Opus audio; there is no single form of compression that WebM implies.