Wikipedia
WebM
WebM is a video file format. It is primarily intended to offer a royalty-free alternative to use in the HTML5 video tag. It has a sister project WebP for images. The development of the format is sponsored by Google, and the corresponding software is distributed under a BSD license.
The WebM container is based on a profile of Matroska. WebM initially supported VP8 video and Vorbis audio streams. In 2013 it was updated to accommodate VP9 video and Opus audio.