DTD

The purpose of a DTD (Document Type Definition) is to define the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes.

The 3 Document Type Definitions

DTD specifies the syntax of a web page in SGML.
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.
XHTML is specified in an SGML document type definition or 'DTD'.
An XHTML DTD describes in precise, computer-readable language, the allowed syntax and grammar of XHTML markup.

There are currently 3 XHTML document types:

STRICT

TRANSITIONAL

FRAMESET

The XHTML standard defines three Document Type Definitions.

The most common is the XHTML Transitional.

Is Mandatory

An XHTML document consists of three main parts:

the DOCTYPE
the Head
the Body

XHTML 1.0

specifies three XML document types that correspond to three DTDs: Strict, Transitional, and Frameset.

XHTML 1.0 Strict

Use this when you want really clean markup, free of presentational clutter. Use this together with Cascading Style Sheets.

XHTML 1.0 Transitional

Use this when you need to take advantage of HTML's presentational features and when you want to support browsers that don't understand Cascading Style Sheets.