XHTML/DTD (Document Type Definition)

Introduction
An XHTML DTD describes the allowed syntax and grammar of XHTML markup. Every XHTML document must start with a DTD declaration and a line of code that declares that you are starting to write XHTML code.

This is the mandatory minimum way to start an XHTML document:

The Three Document Type Definitions
There are currently three XHTML 1.0 document types:
 * Strict
 * Transitional
 * Frameset

XHTML 1.0 Strict
This is used when the user wants a clean markup, free of presentational clutter. Use this in conjunction with Cascading Style Sheets.

XHTML 1.0 Transitional
Use this when taking advantage of XHTML's presentational features and when you want to support browsers that do not understand Cascading Style Sheets. This is the one that is favored to use among developers and is used commonly.

XHTML 1.0 Frameset
Use this when you want to use XHTML Frames to partition the browser window into two or more frames.

XHTML 2.0
XHTML was a working draft, but its development was halted in favor of HTML 5.0. Even though the specification will not be completed, its doctype is: