What is XHTML ?

XHTML is a family of current and future document types and modules that reproduce, subset, and extend HTML 4 [HTML4]. XHTML family document types are XML based, and ultimately are designed to work in conjunction with XML-based user agents. The details of this family and its evolution are discussed in more detail in [<XHTMLMOD].

XHTML 1.0 (this specification) is the first document type in the XHTML family. It is a reformulation of the three HTML 4 document types as applications of XML 1.0 [XML]. It is intended to be used as a language for content that is both XML-conforming and, if some simple guidelines are followed, operates in HTML 4 conforming user agents. Developers who migrate their content to XHTML 1.0 will realize the following benefits:

XHTML documents are XML conforming. As such, they are readily viewed, edited, and validated with standard XML tools.

XHTML documents can be written to operate as well or better than they did before in existing HTML 4-conforming user agents as well as in new, XHTML 1.0 conforming user agents.

XHTML documents can utilize applications (e.g. scripts and applets) that rely upon either the HTML Document Object Model or the XML Document Object Model [DOM].

As the XHTML family evolves, documents conforming to XHTML 1.0 will be more likely to interoperate within and among various XHTML environments.

The XHTML family is the next step in the evolution of the Internet. By migrating to XHTML today, content developers can enter the XML world with all of its attendant benefits, while still remaining confident in their content's backward and future compatibility.

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.

XHTML 1.0 Frameset

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

Most commonly used tags in XHTML