What is DTD?

DTD (Document Type Definition) is a set of markup declarations that define a document type for SGML-family languages (like XML and HTML). It specifies the elements, attributes, and entities that can be used in a document, as well as the relationships between them, thereby providing a grammar or schema for validating the structure and content of documents. DTDs are commonly used to ensure that documents conform to a specific format, enabling consistent processing and exchange of data. However, DTDs have limitations, and XML Schema (XSD) is often preferred for XML validation in modern applications.