DTD purpose — evaluate the statement: “The DTD defines the structure of XML documents.” Decide whether this is correct or incorrect.

Introduction / Context:
A Document Type Definition (DTD) is one of the original schema languages for XML (inherited conceptually from SGML). It specifies the legal building blocks of an XML document—what elements and attributes may appear, in what order, and how many times. This question verifies understanding of the DTD’s role.

Given Data / Assumptions:

• DTD provides grammar-like rules: element content models, attribute lists, entities, notations.
• Other schema languages exist (XSD, RELAX NG), but they do not negate DTD functionality.
• Validation compares an XML instance against the DTD to enforce the structure.

Concept / Approach:
DTDs define structure and permissible attributes but lack strong typing (e.g., cannot easily constrain numeric ranges). Even so, they do define the structure of XML documents. Therefore, the statement is correct, though modern systems often prefer XSD/RELAX NG for richer typing.

Step-by-Step Solution:

Verification / Alternative check:
Validate an XML instance against a DTD with a validating parser; invalid structures are rejected, confirming the DTD’s structural role.

Why Other Options Are Wrong:

• Limiting DTD to HTML is incorrect; DTDs work for arbitrary vocabularies.
• Namespaces/XSLT are unrelated to whether DTDs define structure.

Common Pitfalls:
Confusing well-formedness (XML syntax) with validity (conforming to a DTD or schema); assuming DTDs are obsolete and therefore “incorrect.”

Final Answer:
Correct