As support for WebM is ramping up, Web authors can start using it. However, since not everyone has a WebM-enabled browser, yet, using WebM on your site poses the problem of having to explain to the visitors of your site how they can view WebM. It is inefficient for everyone to have to do this from scratch on their sites. Also, chances are that per-site help text will be incomplete and out of date soon.
To address this problem, with hosting and domain name help from Anne van Kesteren, I have made webm.html5.org as a place to pool the effort. When you publish WebM content, instead of explaining which browsers support WebM, you can simply link to webm.html5.org and it will detect if the user’s browser supports WebM. If the browser doesn’t support WebM, the page will suggest upgrading the browser to a new version that supports WebM, installing a WebM decoder if the browser supports 3rd-party decoders and one is available, switching to another browser or using another operating system (as applicable and in that order).
The dull visual appearance of the page is a known problem. Visual design isn’t my strong point. I have also avoided using logos without permission. If you’d like to contribute nicer CSS or a nicer-looking (but still short and on-topic) test clip, please find hsivonen on the #whatwg IRC channel on Freenode. Also, if you can contribute accurate advice for platforms that aren’t already covered (e.g. FreeBSD, AIX or OS/2), please drop a line on IRC or in the comments here. (You can view source on webm.html5.org to see what is already covered.)