DTD - Document Type Definition

DTD's were derived from SGML and are a component of the XML 1.0 specification. Early on it was clear that DTD's lacked descriptive capabilities, such as data-typing and inheritance that would enable broader implementation of XML.

An optional part of the document prolog, as specified by the XML standard. The DTD specifies constraints on the valid tags and tag sequences that can be in the document. The DTD has a number of shortcomings however, which has led to various schema proposals. For example, the DTD entry <!ELEMENT username (#PCDATA)> says that the XML element called username contains "Parsed Character DATA" -- that is, text alone, with no other structural elements under it. The DTD includes both the local subset, defined in the current file, and the external subset, which consists of the definitions contained in external .dtd files that are referenced in the local subset using a parameter entity.

Data Tree Diagram ou Document Technical Description. Structures types de documents qui décrivent les éléments composant les textes et les règles qui les régissent, html est une déclinaison de la norme SGML avec une DTD très simple.