Potential Competitors

Schematron Schematron uses Extensible Stylesheet Language Transformations to return a report on document structure. While Schematron isn't meant for large-scale machine processing, it can test multiple layers of structure and return author-defined error messages about document structures.
DSD DSD uses a similar set of strategies to Schematron, but is cast in more conventional XML schema syntax.
RELAX RELAX, based on hedge automata theory but still much simpler than XML Schemas, is the latest contender. RELAX is being developed by the Japanese national standards organization, and may end up on a fast-track process through the International Organization for Standardization (ISO).