5.1. Validating and Non-Validating Processors

Conforming XML processors fall into two classes: validating and non-validating. Validating and non-validating processors alike must report violations of this specification's well-formedness constraints in the content of the document entity and any other parsed entities that they read.

Writing a "well-formedness checker," or a non-validating parser that merely checks for well-formedness, is a fairly simple exercise. This is the whole point of the two-tiered (valid documents and well-formed documents) system of conformance: so that certain classes of XML application are very little trouble to develop.