It's called Matroska. And practically everyone promoting Ogg pretends that it doesn't exist.
My theory is that the difference is one of control. Having talked to many Ogg evangelists, this seems to be the problem: Matroska allows you to use video and audio formats that Xiph doesn't like. Ogg does not.
Every Vorbis/Theora video I've
ever seen, which isn't distributed on the web, is contained in Matroska. Ogg is the "default" because of Mozilla, not due to some Xiph conspiracy. There are three reasons Matroska isn't more heavily used:
1. Ogg is the dominant format for free audio. I've tried to use Matroska for storing audio (.mka), but most tools have poor or no support for it. I believe a large part of this is because Matroska's metadata support is even more poorly documented than Ogg's. At least Ogg has some de-facto standards for how metadata should be included; Matroska,̶ ̶a̶s̶ ̶f̶a̶r̶ ̶a̶s̶ ̶I̶ ̶c̶a̶n̶ ̶t̶e̶l̶l̶,̶ ̶i̶s̶ ̶c̶o̶m̶p̶l̶e̶t̶e̶l̶y̶ ̶f̶r̶e̶e̶-̶f̶o̶r̶m̶ ̶(̶a̶n̶d̶ ̶t̶h̶u̶s̶ ̶u̶n̶u̶s̶a̶b̶l̶e̶)̶ has some standard tags, but few libraries can read them. The Matroska developers should sit down and either a) document the existing system or 2) figure out a new one. Then, help get support into common media libraries such as GStreamer or TagLib. Once I can store audio in Matroska, it's easier to encourage its use on the web.
2. Mozilla doesn't like depending on platform libraries, even if they're cross-platform, since history has shown that they tend to be more trouble than they're worth. Look at the Mozilla C++ style guide; half of it is dedicated to avoiding C++ features as simple as namespaces or exceptions because some obscure platforms don't support them well. Therefore, they won't support GStreamer or QuickTime, and support for any format has to be included in the Mozilla repository directly.
3. Since #1 means Ogg must be supported, and #2 means there's a huge disincentive to support more than the bare minimum of formats, Matroska is seen as too much trouble to support. I think this won't change until more users request it.
For what it's worth, here's Mark Pilgrim's opinion on Matroska, after I requested at least a sentence or two be included in his new HTML 5 book (which covers such unused fossils as AVI and XVid):
> Could you add a paragraph on the Matroska[1][2] container format to
> the list of containers?
I could, but no browser supports it (even those that support
Theora-in-Ogg), so there's little point. Then again, I do mention AVI,
which no browser supports either, but that's mostly because I think
most readers would be familiar with it. Few people have heard of MKV
except Free Software hippies and movie pirates. :-)
If it actually lands in Firefox or some other browser, I'll revisit
this decision.