This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
A Document Type Definition (DTD) is the definition of a specific type of electronic document. Specifically, it defines (1) a vocabulary of element names and attribute names, (2) guidelines concerning permitted relationships between elements, (3) specifications for required and optional items, and (4) a compliant XML-based structure in accord with the rules specified in SGML (ISO 8859:1986) and/or XML specifications. XML is a simplified version of SGML.
You can create a DTD in a simple text editor such as UltraEdit once you understand the rules.