Reference · Containers
What is WebM?
WebM is an open multimedia container derived from a constrained subset of Matroska and designed for predictable web media delivery.
WebM is: a container. WebM is not: VP9, VP8, AV1, Opus or Vorbis; those are codecs used within WebM profiles.
Typical WebM codecs
| Track type | Common WebM codecs |
|---|---|
| Video | VP8, VP9 and modern AV1 deployments where supported |
| Audio | Vorbis or Opus |
| Container family | Matroska-derived, with a deliberately constrained web-oriented profile |
What WebM keeps—and what it leaves out
WebM keeps Matroska's track-based structure, allowing many tracks, track flags, chapters and tags. But WebM guidelines intentionally reject some broader Matroska features to make browser/player behaviour more predictable.
- Multiple media tracks: supported.
- Default/forced track flags: supported.
- Chapters: supported in the WebM subset.
- Tags: supported in a restricted form.
- General Matroska Attachments: not part of the WebM profile.
- Codec choices: deliberately constrained compared with ordinary MKV.
WebM vs MKV
WebM is based on Matroska but is intentionally more restrictive. MKV is appropriate when broad codec, attachment and presentation flexibility matters; WebM prioritises interoperable web delivery.
The .webm extension still does not tell you the codec
Two .webm files can use different video or audio codecs. If a device says it supports “WebM,” you still need to know which codecs, profiles, bit depths and features it actually decodes.