1. more than one document type definition

2. error occurred while parsing document type definition

3. Schema languages Document Type Definition (DTD) and XML Schema (W3C) were included.

4. The extensible markup language is stored in a document type definition file (DID).

5. The oldest schema language for XML is the Document Type Definition (DTD), inherited from SGML.

6. The simplest schema language is the Document Type Definition (DTD), which is part of the basic XML specification.