Non validating parser xml
W3C XML Schema is complex and hard to learn, although that's partially because it tries to do more than mere validation (see PSVI).
Again, this design is intentional and is to separate validation and augmentation.
All XML documents must be well-formed, but it is not required that a document be valid unless the XML parser is "validating", in which case the document is also checked for conformance with its associated schema.
DTD-validating parsers are most common, but some support XML Schema or RELAX NG as well.
Unlike W3C XML Schema, RELAX NG was designed so that validation and augmentation (adding type information and default values) are separate.
W3C XML Schema has a formal mechanism for attaching a schema to an XML document, while RELAX NG intentionally avoids such mechanisms for security and interoperability reasons.