The DTD (Document Type Definition) is the only kind of validation that's really built in into XML.
Drawbacks are, that it lacks data type checking, and the XRC file format is not very DTD-friendly.
Here's a first sketch for a DTD, it's under construction and of limited use:
Remarks
- Doctype declaration must be added to XRC file (
<!DOCTYPE resource SYSTEM "../dtd/xrc.dtd">) - No data type validation
- Classes are kept in object's class attributes: makes it difficult (impossible?) to restrict children elements