What is a DTD? What DTD do you generally use? Why? Pros and cons.

A DTD is a Document Type Definition, also know as DOCTYPE. In a document served as text/html, the DOCTYPE informs the browswer how to interpret the content of the page. If the the doctype is not declared, the browser assumes you don’t know how to code, and goes into “quirks mode”. If you know what you are doing and include a correct XHTML DOCTYPE, your page will be rendered in “standards mode”.