Document type definition

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 Document Type Definition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes. A DTD can be declared inline inside an XML document, or as an external reference.

XML uses a subset of SGML DTD.

As of 2009^[update], newer XML namespace-aware schema languages (such as W3C XML Schema and ISO RELAX NG) have largely superseded DTDs. A namespace-aware version of DTDs is being developed as Part 9 of ISO DSDL. DTDs persist in applications that need special publishing characters, such as the XML and HTML Character Entity References, which derive from larger sets defined as part of the ISO SGML standard effort.

A DTD is associated with an XML or SGML document by means of a document type declaration (DOCTYPE). The DOCTYPE appears in the syntactic fragment doctypedecl near the start of an XML document. The declaration establishes that the document is an instance of the type defined by the referenced DTD.

DTDs make two sorts of declaration:

The declarations in the internal subset form part of the DOCTYPE in the document itself. The declarations in the external subset are located in a separate text file. The external subset may be referenced via a public identifier and/or a system identifier. Programs for reading documents may not be required to read the external subset.

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