Install each builtin validation alone on a blank validator, run it
against a seeded subject, and assert the resulting errors by count,
reason, and coding path, matching the validation-rules recipe.
- Structural.uniqueAttributes (multi-error, one per duplicate).
- DTD.contentModel, DTD.attributeDeclarations (located reasons).
- DTD.undeclaredElement (the single-error Bool form: Failed to satisfy).
- DTD.identifierIntegrity (tree-wide, located at the document root).
- XSD.contentValidity (located at the document element).
