A Document Type Definition (DTD) is a specification file that contains a set of rules or declarations that define the structure and content of an XML or SGML document¹. A DTD can be used to validate the syntax and semantics of an XML or SGML document, ensuring that it conforms to the expected format².

A DTD can be declared either inline inside an XML or SGML document, or as an external reference to a separate file¹. The syntax for declaring a DTD is as follows³:

• The root-element is the name of the root element of the document.

• The DTD-identifier is either a system identifier (a URI) or a public identifier (a name) that points to the location of the external DTD file, or both. If the DTD is inline, the DTD-identifier is omitted.

• The internal-subset is an optional set of declarations that are included in the document itself, and override or supplement the external DTD file.

A DTD consists of a series of markup declarations that define the elements, attributes, entities, notations, and processing instructions that are allowed in the document¹. The syntax for markup declarations is as follows³:

• The <!ELEMENT> declaration defines the name and content model of an element. The content model specifies what child elements and text data are allowed inside the element. For example, <!ELEMENT note (to,from,heading,body)> defines an element named note that must contain four child elements: to, from, heading, and body.

• The <!ATTLIST> declaration defines the name, type, and default value of an attribute for a given element. For example, <!ATTLIST note id ID #REQUIRED> defines an attribute named id for the note element, which must be of type ID and must be specified in the document.

• The <!ENTITY> declaration defines the name and value of an entity, which is a placeholder for some text or markup that can be referenced in the document. For example, <!ENTITY author "John Doe"> defines an entity named author with the value "John Doe".

• The <!NOTATION> declaration defines the name and value of a notation, which is a way to identify the format or encoding of some external data that is referenced in the document. For example, <!NOTATION gif SYSTEM "image/gif"> defines a notation named gif with the system identifier "image/gif".

• The <?processing-instruction?> declaration defines a processing instruction, which is a way to pass some information or instructions to the application that processes the document. For example, <?xml-stylesheet href="style.css"?> defines a processing instruction that instructs the application to use the style sheet "style.css".