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.
Appendix A contains the complete DTD specification. The DTD is also available in the Developer's Toolkit. It is located in the /usr/dt/dthelp/dthelptag/dtd directory and is named helptag.dtd.
dthelptagdtd(4)man page