What is DTD (Document Type Definition) and what role does it play in XML?

DTD describes the structure of an XML document, defining permissible elements, attributes, their order, and nesting.

It provides validation of XML files by checking their compliance with specified rules. This helps ensure the correctness of data format and their processing capabilities.

Role of DTD:

• Defining structure: Specifies a set of allowed elements and their hierarchy.
• Defining attributes: Indicates permissible attributes and their types for each element.
• Defining content of elements: Regulates whether an element can contain text, other elements, or be empty.
• Validation: Used by parsers to verify compliance of the XML document with the described rules.

Example of defining an element and attribute in DTD:

<!ELEMENT book (title, author, year)>
<!ATTLIST book
  id CDATA #REQUIRED>

Here, the book element is defined with child elements title, author, year, and a mandatory attribute id.