The sister video project of the WebP image format is WebM.
WebP and WebM emerged from Google’s effort to provide open, web-oriented media formats. WebP is designed primarily for still or animated images, while WebM is a container for audiovisual media, especially video delivered through the web. Their names and development histories are closely linked, but they are not interchangeable file types.
WebP’s lossy compression is derived from VP8 intra-frame technology, while WebM originally paired VP8 video with Vorbis audio. Later WebM implementations also supported newer codecs such as VP9 and AV1, along with audio formats including Opus. WebM is based on a constrained form of the Matroska container.
A common mistake is choosing VP9 because it is strongly associated with WebM. VP9 is a video codec, not the project or container name. Theora is another video codec, and AVIF is an image format. WebM is the matching sister project because it addresses moving audiovisual content alongside WebP’s image-focused role.