What Mike said is correct as for WEBM.
There is also another issue. That is a Google item these days and they are using it widely on YouTube, but the better / more current versions are not widely available to us normal folks.
Version 8, that Codec is in many programs
Version 9, not so many - seems only the places that render online for a fee have it. Even many of the Plug-ins they link to are the VP8 version last I checked in the spring.
The WebM Project | VPx and WebM Tools
I subscribe to a streaming media forum and those with more experience than me talk about it as being good, as well as MPEG-DASH and others...
The problem is getting everybody onboard. It took YEARS and YEARS for h.264 in the MP4 container to be so widespread as to be pretty much universal. Not sure if people want to take on that task again and again with each new codec.