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A document type definition (DTD) is a specification file that contains set of markup declarations that define a document type for an SGML-family markup language (GML, SGML, XML, HTML). The DTD specification file can be used to validate documents. A DTD defines the valid building blocks of an XML document.
This article lists the character entity references that are valid in HTML and XML documents. A character entity reference refers to the content of a named entity. An entity declaration is created in XML, SGML and HTML documents (before HTML5) by using the <!ENTITY name "value"> syntax in a Document type definition (DTD).
W3C XML Schema is complex and hard to learn, although that is partially because it tries to do more than mere validation (see PSVI). Although being written in XML is an advantage, it is also a disadvantage in some ways. The W3C XML Schema language, in particular, can be quite verbose, while a DTD can be terse and relatively easily editable.
A document type declaration, or DOCTYPE, is an instruction that associates a particular XML or SGML document (for example, a web page) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML 2.0 - 4.0). [1]
Two peculiar features that distinguish DTDs from other schema types are the syntactic support for embedding a DTD within XML documents and for defining entities, which are arbitrary fragments of text or markup that the XML processor inserts in the DTD itself and in the XML document wherever they are referenced, like character escapes.
A valid XML document is defined in the XML specification as a well-formed XML document which also conforms to the rules of a Document Type Definition (DTD). According to JavaCommerce.com XML tutorial, "Well formed XML documents simply markup pages with descriptive tags. You don't need to describe or explain what these tags mean.
XML does not use the grammar (DTD) to change delimiter maps or to inform the parse modes, and does not allow tag omission; consequently, XML validation of elements is not active in the sense that SGML validation is active. SGML without a DTD (e.g. simple XML), is a grammar or a language; SGML with a DTD is a metalanguage.
XML Schema, published as a W3C recommendation in May 2001, [2] is one of several XML schema languages. It was the first separate schema language for XML to achieve Recommendation status by the W3C.