DTD

Last updated April 23, 2026

DTD (Document Type Definition) is the original schema language for XML, inherited from SGML. A DTD defines what elements a document may contain, what attributes those elements may have, and what entities are declared. It was the only validation option in the earliest XML specs and predates XSD by several years.

Example

<!DOCTYPE book [
  <!ELEMENT book (title, author+)>
  <!ELEMENT title (#PCDATA)>
  <!ELEMENT author (#PCDATA)>
  <!ATTLIST book id CDATA #REQUIRED>
  <!ENTITY copy "&#169;">
]>

<book id="978">
  <title>Deep Work</title>
  <author>Cal Newport</author>
</book>

The <!DOCTYPE> block declares a grammar for <book> elements: they must contain exactly one <title> followed by one or more <author>s, and they must have a required id attribute.

Why DTDs are largely superseded

XSD replaced DTDs for most new XML work because it addresses three problems:

DTDs have their own syntax, not XML. XSDs are themselves XML documents, which means XML tooling applies uniformly.
DTDs have weak typing. Attribute values are essentially strings, with a few restricted subtypes like ID and IDREF. XSDs provide rich typing (dates, numbers, base64 binary, etc.).
DTDs cannot support namespaces. This is disqualifying for any modern XML workflow.

Where you still encounter DTDs

HTML documents with a historical DOCTYPE (<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" ...>). Modern HTML5 uses a simpler DOCTYPE with no DTD body.
Publishing formats like DocBook originally used DTDs; many projects have migrated to XSD or RELAX NG, but the DTD versions are still in circulation.
Email standards (e.g., DKIM-associated specs).
Legacy industry document types — old versions of SEC filings, for example.

Entity declarations

One feature DTDs have that XSD does not is entity declaration. You can define © once in the DTD and use it throughout the document as shorthand for ©. This is how HTML gets its  , &, and hundreds of other named entities. The cost is that entities also enable denial-of-service attacks (the “billion laughs” attack), which is why modern XML parsers disable external entity loading by default.

Should I use a DTD in 2026?

Almost never for new work. Use XSD or JSON Schema depending on your format. The only good reasons to write a DTD today are (a) you are publishing to a format that still requires one, or (b) you are extending an existing XML vocabulary that is DTD-based.

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