Document Type Definition
Document Type Definition (DTD) is a formal specification used in XML and SGML to define the structure, elements, and attributes of a document type. It specifies the legal building blocks of an XML document, including element declarations, attribute lists, and entity references, enabling validation against a defined schema. DTDs help ensure consistency and interoperability in data exchange by providing a blueprint for document structure.
Developers should learn DTD when working with legacy XML-based systems, such as in publishing, configuration files, or data interchange formats like XHTML, where validation against a predefined structure is required. It is particularly useful for ensuring data integrity in applications that rely on strict document formats, such as in legal or scientific domains, though modern alternatives like XML Schema are often preferred for new projects due to richer features.