WebM is an open, royalty-free video format that Google launched in 2010 for HTML5 video on the web. It’s based on the Matroska (MKV) container but limited to a small set of open codecs, so any browser can rely on playing it without patent licences.
What’s inside a WebM
| Stream | Codecs | Notes |
|---|---|---|
| Video | VP8 | The original WebM codec, now mainly in older files |
| Video | VP9 | The common choice; files 30–50% smaller than H.264 at similar quality |
| Video | AV1 | Newest and most efficient; slower to encode |
| Audio | Vorbis, Opus | Opus is the modern default |
WebM can also hold video with transparency (an alpha channel), which is used for animated stickers and overlays on websites.
Where WebM comes from
- Screen and meeting recorders that run in the browser, because browsers record WebM natively.
- Websites serving efficient video, including YouTube’s VP9 streams.
- Downloads of web videos and animations.
Where WebM doesn’t play well
Browsers handle WebM well: Chrome, Edge, Firefox and Opera everywhere, and Safari on recent macOS and iOS versions. Outside the browser, support is patchy. QuickTime and iMovie on Mac, the iPhone Photos app, PowerPoint, many TVs and many video editors don’t accept it, which is why WebM files are often converted to MP4 before sharing or editing.
WebM or MP4?
On your own website, WebM with VP9 gives smaller files and faster loading, with an MP4 fallback for any browser that can’t play it. For sending to people, uploading to social networks or editing, MP4 with H.264 is the compatible choice.