DTD
DTD (Document Type Definition) is a markup language schema that defines the structure, elements, and attributes of an XML or SGML document. It specifies the rules for how elements can be nested and what data they can contain, enabling validation and consistency in document processing. DTDs are commonly used in older XML-based systems, web standards like HTML, and document management applications.
Developers should learn DTD when working with legacy XML systems, validating document structures in applications like publishing or data exchange, or maintaining compatibility with standards such as XHTML. It is useful for ensuring data integrity in scenarios where XML schemas are not required, but its use has declined in favor of more modern alternatives like XML Schema (XSD) for complex validations.