DTD - Why Use DTD

A DTD (Document Type Definition) is used to define the structure, legal elements, and attributes of an XML document.

Here’s why you might use one:

1. Validation –
   A DTD specifies exactly what tags can appear, in what order, and with what attributes.
   XML parsers can check documents against the DTD to ensure they follow the agreed structure.

2. Consistency –
   If multiple people or systems produce XML for the same purpose (e.g., invoices, product catalogs), the DTD acts like a rulebook so everyone’s XML looks the same.

3. Interoperability –
   Systems exchanging XML can rely on the DTD to know what to expect, making integration easier.

4. Documentation –
   It serves as a human-readable blueprint of the XML structure, making it easier for developers to understand and maintain.

5. Backward Compatibility –
   If you keep the same DTD, older systems can still parse and validate new XML files without changes.

That said, modern XML often uses XML Schema (XSD) instead of DTDs because XSDs are more powerful, typed, and written in XML itself.
But DTDs are simpler, lighter, and still good for small projects or legacy systems.