Editing XML (section)

=== Document type definition ===
{{Main|Document type definition}}
The oldest schema language for XML is the [[document type definition]] (DTD), inherited from SGML.

DTDs have the following benefits:
* DTD support is ubiquitous due to its inclusion in the XML 1.0 standard.
* DTDs are terse compared to element-based schema languages and consequently present more information in a single screen.
* DTDs allow the declaration of [[SGML entity|standard public entity sets]] for publishing characters.
* DTDs define a ''document type'' rather than the types used by a namespace, thus grouping all constraints for a document in a single collection.

DTDs have the following limitations:
* They have no explicit support for newer [[feature (software design)|features]] of XML, most importantly [[XML Namespace|namespaces]].
* They lack expressiveness. XML DTDs are simpler than SGML DTDs and there are certain structures that cannot be expressed with regular grammars. DTDs only support rudimentary datatypes.
* They lack readability. DTD designers typically make heavy use of parameter entities (which behave essentially as textual [[macro (computer science)|macros]]), which make it easier to define complex grammars, but at the expense of clarity.
* They use a syntax based on [[regular expression]] syntax, inherited from SGML, to describe the schema. Typical XML APIs such as [[Simple API for XML|SAX]] do not attempt to offer applications a structured representation of the syntax, so it is less accessible to programmers than an element-based syntax may be.

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.

DTD technology is still used in many applications because of its ubiquity.