Sustainability of Digital Formats
Planning for Library of Congress Collections
Sustainability of Digital Formats
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | XML Document Type Definition (DTD) |
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| Description | An XML Document Type Definition is a formal expression (in XML) of the structural constraints for a class of XML documents. The DTD language constructs are element and attribute-list declarations. Element declarations name the elements allowed within a document of this type, and specify whether and how declared elements and runs of character data may be contained within each element. Attribute-list declarations name the permitted attributes for each declared element, including the type of each attribute value, if not an explicit set of valid value(s). A DTD is associated with an XML document via a Document Type Declaration, which is a tag that appears near the start of the XML document. The declaration establishes that the document is an instance of the type defined by the referenced DTD. |
| Relationship to other formats | |
| Subtype of | XML, Extensible Markup Language |
| LC experience or existing holdings | LC has developed and/or used various SGML and XML DTDs since the mid 1990s. Examples include: the American Memory (AMMEM) SGML DTD used for converted textual materials; the EAD Document Type Definition (DTD), a standard for encoding archival finding aids (originally SGML, now in an XML version); and the NCBI/NLM Journal Archiving and Interchange DTD. |
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| LC preference |
| Disclosure | The language for expressing DTDs is defined within the specification of XML. |
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| Documentation | Extensible Markup Language (XML) 1.0 |
| Adoption | Widely adopted for specification of document-centric XML document types and use in publishing. The various versions of HTML are defined through DTDs. See list of HTML and XHTML Document Type Definitions at University of Toronto. Examples of other DTDs in widespread use within particular communities include News Industry Text Format (NITF), DTB (Digital Talking Book), and the NCBI/NLM Journal Archiving and Interchange DTD. |
| Licensing and patents | None |
| Transparency | See XML. |
| Self-documentation | Comments as to purpose, structure, etc. can be embedded in DTD files. |
| External dependencies | None |
| Technical protection considerations | None |
| Text | |
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| Normal rendering | DTDs can be viewed and searched as normal text.
DTDs are not strictly "normal" text, since their function is to support the creation, validation, and manipulation of documents in the class they define. Quality and functionality factors are typically assessed on the basis of a particular DTD and the fitness of the defined document class for the intended purpose. |
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| Format specification | A DTD has the special purpose of defining a class of XML documents. |
| Tag | Value | Note |
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| Filename extension | dtd |
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| Internet Media Type | application/xml-dtd |
See RFC 3023 for further details. |
| General | As of March 2008, several document types formerly specified only as DTDs now also have specifications in the XML Schema or Relax NG languages. |
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| History | Derived from SGML Document Type Definition specification language. |
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