DTDs and Doctypes in HTML, XML, and XHTML

“So What is a DTD?

The tags in the HTML language must be defined somewhere, right? They are defined in the DTD, which stands for Document Type Definition. In a DTD for HTML, all the HTML tags are defined. Everything is defined in the DTD — the tags, the attributes, the possible values they can hold. Think of the DTD as the HTML tag dictionary: it lists all the tags, what they mean, and how they relate to each other.

Also, there are different DTDs because there are different versions of HTML:

• HTML 4.01 Strict
• HTML 4.01 Transitional
• XHTML 1.0 Strict
• XHTML 1.0 Transitional
• and several more. “

Read thefull tutorial from DHTMLdev.com. This document is available in PDF format for saving and reference.