Most authors use an SGML document type definition to define a document's class.
The problem is that DTD s can only define document syntax rules. DTDs cannot define the semantics of a document. In order to properly define a document class, authors should use an SGML architecture. Authors can declare that their documents conform to an SGML architecture by adding either an architecture notation, or an architecture processing instruction to their document. The HTML 4.0 specifications force the HTML 4.0 DTD to define document semantics by stating that authors are not allowed to modify the document type definition.