WebP is a still image format, described in the WebP Container Specification. It is a container for VP8 Lossy or VP8 Lossless.
The “Making Software - Image Compression” book chapter also has a good overview of a number of image formats, including WebP.
There are two WebP variants: lossy (based on VP8) and lossless (based on what‘s essentially an unrelated design, other than “WebP” branding). Wuffs’ VP8 implementation is in a separate package, as the WebM moving image format also uses VP8. There's no Wuffs WebM package yet but there might be in the future.