XML namespaces are inherently vendor-neutral, and as such, anyone can define one, even me. So in order to help standardize XML dialects that do not define a namespace, I have put together a list of document types that really should have a namespace, but have not taken the time to define one.
There really isn't much to say about these. Some of them are based on resources which describe the XML dialects, whereas others (like x3d-namespace) are a version-neutral namespace (while some X3D implementors have used x3d-3.0.xsd or x3d-3.2.xsd, for which files actually exist). If anyone thinks some of these are poor choices, then let me know, and I'll change it.