Document Types

There are several SGML and XML schema languages–an XML schema is a description of a type of XML document, typically, expressed in terms of constraints on the structure and content of documents of that type–and the DTD or Document Type Definition is one of these. Primarily, a DTD is used to express a schema through a set of declarations that conform to particular mark-up syntax where class, or type, of document, is described. Furthermore, constructs that are not always required to establish document structure, but that may affect the interpretation of some documents can be declared by a DTD.

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Furthermore, a subset of DTD which imposes various restrictions on the structure of the document describes XML documents. This is required per the XML standard since XML is in itself an application of SGML optimized for automated parsing. It is important for documents to adhere to the general rules of XML since the general syntax of XML schema languages is rigid. This will ensure the readability and understanding the arrangement of all XML-aware software. Lastly, a valid XML document complies with a particular schema/DTD, in addition to being well-formed.