webm
What is a .webm file?
video/webm
WebM is a deliberately narrow profile of Matroska, restricted to codecs anyone may implement royalty-free. It keeps Matroska's EBML structure but permits only VP8, VP9, or AV1 video with Vorbis or Opus audio, which is what made it the open counterpart to MP4 in an HTML5 `<video>` element. Google announced it at I/O in 2010 alongside the newly open-sourced VP8 and added VP9 and Opus in 2013. That restriction is also its limit: a WebM cannot carry H.264 or AAC at all.
How to use a .webm file
Use an example WebM to test HTML5 `<video>` playback, EBML demuxing, and VP8/VP9 decoding — checking that a converter turning an MP4 into WebM re-encodes rather than attempting a stream copy, since neither H.264 nor AAC has any place in the container.
Download example .webm files
- WebM — VP9 ClipThe same clip as WebM/VP9 — the royalty-free web video format. Video-only; for testing HTML5 <video>, VP9 decoding, and WebM conversion.
- WebM — VP9 Short ClipShort VP9 WebM SAMPLE companion for container conversion tests.
- Codec — VP9The shared pan-city base plate encoded with VP9 (libvpx-vp9). Google's royalty-free predecessor to AV1, and still the workhorse of WebM delivery. Plays natively in every major browser. Every clip in this group carries the identical picture, so a decoder-support matrix built from them isolates the codec as the only variable.
- Codec — VP8The shared pan-city base plate encoded with VP8 (libvpx). The first WebM codec, now legacy but still what MediaRecorder emits by default in several browsers — so it turns up in user-generated uploads far more often than its age suggests. Every clip in this group carries the identical picture, so a decoder-support matrix built from them isolates the codec as the only variable.
- Alpha Channel — VP9 in WebMVideo with a real per-pixel alpha channel: VP9 yuva420p in WebM, the one alpha path browsers decode natively. Composite it over a page background and the transparency is genuine, not a chroma key. Roughly 90% of each frame is fully transparent. Two traps this file exists to expose. First, auto-alt-ref must be disabled at encode time or libvpx silently drops the alpha plane, producing a valid file with no transparency and no error. Second, WebM stores VP9 alpha in BlockAdditional and signals it with AlphaMode=1, so FFmpeg's NATIVE vp9 decoder reports pix_fmt yuv420p and decodes fully opaque — you must force `-c:v libvpx-vp9` to see the alpha at all. Probing this file with default settings and concluding it has no alpha is the expected mistake.
- Alpha Channel — Opaque VP9 TwinThe same codec and container with NO alpha plane, as the control for the alpha clip in this group. Useful for checking that alpha detection reads the pixel format rather than assuming every WebM is transparent — and for confirming a compositing bug is in the alpha handling rather than in the player.
- Multi-Track Subtitles — Embedded WebVTT (WebM)WebVTT muxed inside a WebM container, which the Matroska specification allows and almost nothing consumes. No browser surfaces an embedded WebVTT track through the HTML TextTrack API — the web platform expects a separate <track> element pointing at a sidecar file — so this is the fixture for the gap between what a container is permitted to hold and what a player will actually give you. One caveat about this file specifically. Every other fixture in this phase carries byte-identical H.264 video, but the WebM muxer accepts only VP8, VP9 or AV1 video and WebVTT subtitles — that restriction is the container's definition, not a limitation of the tooling — so the picture here is re-encoded to VP9 from the same source frames. Same content, different bytes.
- Intentionally Corrupt — WebM Truncated Mid-StreamAn intentionally corrupt WebM, VP9 in Matroska, cut to 45% of its bytes. Not a valid file by design. The browser-facing member of this group. A truncated WebM starts playing in an HTML5 <video> element and then fires an `error` event mid-stream, which is a genuinely awkward state to handle: your player has already reported success, already hidden the spinner, and already told the user the duration. Use it to check that the error path is wired to something more useful than a frozen frame.
and 40 more in the library.