Why JAXX can't have a schema

From JAXXWiki

If you take a look at the competition, you'll see that most of them have schemas or DTDs which define their legal tags and attributes. Schemas are very helpful, so why doesn't JAXX have one?

Simply put, because it can't. A schema or DTD defines which tags are legal and which are not. Competitive tools support a limited set of tags, defined by their authors, and so can provide a precise list of what their tools support. JAXX class tags, on the other hand, can load any class, not just a small predefined set of them. Any tag corresponding to a Java class available at compile time will work, and there is no way to express that in a schema or DTD.

This should not be construed as a strike against JAXX; if anything, it is a comment on how limited schemas and DTDs are.