video/webm

Google's royalty-free answer to MP4: a trimmed-down Matroska profile that every browser plays without licensing anxiety. The type says container, never codec — video/webm reveals nothing about whether VP8, VP9, or AV1 is inside, which is exactly what the codecs parameter exists to answer (video/webm; codecs="vp9, opus"). The EBML magic at offset 0 is byte-identical to a .mkv file's; the DocType element a few bytes in is what actually says webm, so sniffers that stop at the signature will call every Matroska file WebM.

Defined by WebM Container Guidelines. Registry facts from the IANA media-types registry via mime-db.