A Document Type Definition (DTD) defines a set of elements to create a structured (or hierarchical) document. The DTD specifies the syntax for each element and governs how and where elements can be used in a document.
The Helptag 1.3 DTD tag set and its associated rules are referred to as formal markup. The DTD conforms to the Standard Generalized Markup Language (SGML) ISO specification 8879:1986. This means that you can use formal markup to create help files that are SGML compliant.