XSD 1.0 versus XSD 1.1 for portable schema assertions

Goal is to constrain XML documents with namespaces, typed values and co-occurrence rules while keeping validation portable across common XML parsers.

DTD is built into XML 1.0 and widely supported, but it lacks namespace awareness and offers only coarse typing. XSD provides typed elements and attributes, occurrence constraints and namespace support, with schemas that are more verbose and heavier to validate.

XSD 1.1 adds assertions and conditional type assignment beyond XSD 1.0. Many widely deployed validators implement only XSD 1.0, so the choice between richer constraints and broad portability is unresolved for a design that must validate safely and run on multiple runtimes.

Is use of XSD 1.1 features compatible with validators limited to XSD 1.0? Which portability constraints apply when assertions are needed for business rules?