Ok, before we continue lets's pick apart our template a little.

view plain print about
1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
3<html xmlns="http://www.w3.org/1999/xhtml">
4    <head>
5        <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
6        <title>Untitled Document</title>
7    </head>
9    <body>
11    </body>

We defined the doctype of our page as a transitional XHTML 1.0 document with english as the language. The doctype declaration is a required element of any well-formed XHTML page, and references the appropriate DTD (document type definition) for the content of the document.

From W3Schools:

DTD is used by SGML applications, such as HTML, to specify rules that apply to the markup of documents of a particular type, including a set of element and entity declarations.