These browsers support WebM on Windows, Mac and Linux:
WebM is a format for HTML5 video that is designed to be high-quality and royalty-free*.
HTML5 video makes video a built-in part of the Web platform the way images have been already.
Royalty-free* means that Web sites do not need to pay royalties for serving WebM video for viewing (regardless of the business model of the site) and that distributors of WebM-enabled software do not need to pay royalties for supporting WebM in their software and can instead use their resources for creating great software for you.
You have probably been pointed to this site by another site that uses WebM video, so you need WebM support to view videos on that site.
* No known royalty claims at the time of writing.
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