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A Document Type Definition (DTD) is a set of markup declarations that define a document type for an SGML-family markup language (SGML, XML, HTML).

A DTD uses a terse formal syntax that declares precisely which elements and references may appear where in the document of the particular type, and what the elements’ contents and attributes are. A DTD can also declare entities which may be used in the instance document.


XML schema

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, above and beyond the basic syntactical constraints imposed by XML itself. These constraints are generally expressed using some combination of grammatical rules governing the order of elements, Boolean predicates that the content must satisfy, data types governing the content of elements and attributes, and more specialized rules such as uniqueness and referential integrity constraints.

There are languages developed specifically to express XML schemas. The Document Type Definition (DTD) language, which is native to the XML specification, is a schema language that is of relatively limited capability, but that also has other uses in XML aside from the expression of schemas. Two more expressive XML schema languages in widespread use are XML Schema (with a capital S) and RELAX NG.

XML DTDs and schema validation

The XML DTD syntax is one of several XML schema languages. However, many of the schema languages do not fully replace the XML DTD. Notably, the XML DTD allows defining entities and notations that have no direct equivalents in DTD-less XML (because internal entities and parsable external entities are not part of XML schema languages, and because other unparsed external entities and notations have no simple equivalent mappings in most XML schema languages).

Most XML schema languages are only replacements for element declarations and attribute list declarations, in such a way that it becomes possible to parse XML documents with non-validating XML parsers (if the only purpose of the external DTD subset was to define the schema). In addition, documents for these XML schema languages have to be parsed separately, so validating the schema of XML documents in pure standalone mode is not really possible with these languages: the document type declaration remains necessary for at least identifying (with a XML Catalog) the schema used in the parsed XML document and that will be validated in another language.

A common misconception holds that a non-validating XML parser does not have to read document type declarations, when in fact, the document type declarations must still be scanned for correct syntax as well as validity of declarations, and the parser must still parse all entity declarations in the internal subset, and substitute the replacement texts of internal entities occurring anywhere in the document type declaration or in the document body.

A non-validating parser may, however, elect not to read parsable external entities (including the external subset), and does not have to honor the content model restrictions defined in element declarations and in attribute list declarations.

If the XML document depends on parsable external entities (including the specified external subset, or parsable external entities declared in the internal subset), it should assert standalone="no" in its XML declaration. Identification of the validating DTD may be performed by the use of XML Catalogs, in order to retrieve its specified external subset.

In the example below, the XML document is declared with standalone="no" because it has an external subset in its document type declaration:


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