DTD

Document Type Definition (DTD) is a markup language used to define the structure of an XML document. It specifies the legal elements, attributes, and their relationships, ensuring that XML documents are well-formed and consistent. DTDs use a simple syntax, distinct from XML, starting with <!DOCTYPE. Elements are declared with <!ELEMENT, attributes with <!ATTLIST, and entities with <!ENTITY. For example, a DTD might define that a "book" element must contain "title" and "author" elements, both of which can contain text data.

While DTDs are straightforward and widely supported, they lack advanced features like data typing, which are available in XML Schema. They also offer limited support for namespaces and are less suitable for complex document structures. Despite these limitations, DTDs remain popular due to their simplicity and effectiveness for basic validation tasks.

Here is an example of a simple DTD:

<!DOCTYPE book [

<!ELEMENT book (title, author)>

<!ELEMENT title (#PCDATA)>

<!ELEMENT author (#PCDATA)>

]>

This DTD specifies that a "book" must contain "title" and "author" elements, both of which hold text content.