Section 8.3. Introduction to XHTML

8.3. Introduction to XHTML

With the finalization of the XML Recommendation in hand (see Chapter 7), the W3C had a streamlined and web-friendly standard for defining markup languages. It should come as no surprise that one of the top priorities was reformulating HTML (an SGML application) into an XML application. XHTML is the result.

XHTML 1.0 contains the same list of elements and attributes as HTML 4.01. It even has the same three associated DTDs: Strict, Transitional, and Frames. The difference is that, as for all XML applications, correct syntax is suddenly critical. So while browsers are forgiving of a certain amount of looseness in HTML, XHTML documents are required to be well-formed. The syntax requirement differences between HTML and XHTML are listed in the upcoming "Well-Formed XHTML" section. The W3C recognizes the benefit of having a stricter professional standard and a more relaxed standard that is accessible to anyone who wants to publish on the Web, so both HTML and XHTML standards are currently maintained and supported by current browsers.

8.3.1. The XHTML Family

XHTML is not just one, but a family of document types. Between January 2000 and June 2001, the W3C turned out four XHTML Recommendations: XHTML 1.0, XHTML Basic, the Modularization of XHTML, and XHTML 1.1. They are currently reviewing XHTML 2.0 and XHTML-Print, both based on modular XHTML. This section takes a brief look at each one. You can find detailed and up-to-date information on the W3C site at w3c.org/MarkUp. (For example, on May 27, 2005, the seventh working draft of XHTML 2.0 was published.)

8.3.1.1. XHTML 1.0

The XHTML 1.0 Recommendation (released in January 2000) is just a reformulation of the HTML 4.01 specification according to the rules of XML. Like HTML 4.01, XHTML 1.0 comes in three varietiesStrict, Transitional, and Frameseach defined by a separate Document Type Definition (DTD). These are discussed in the next section.

8.3.1.2. The Modularization of XHTML

In a world where HTML content is being used on devices as varied as cell phones, desktop computers, refrigerator panels, dashboard consoles, and more, a "one-size-fits-all" content markup language will no longer work. Modularization is the solution to this problem. Instead of one comprehensive set of elements, this Recommendation defines a way to break XHTML into task-specific modules. A module is a set of elements that handle one aspect or type of object in a document.

Modularization is the way of the future for markup standards. This approach has a number of benefits:

• Special devices and applications can mix and match modules based on their requirements and restraints.

• It prevents spin-off, device-specific languages. Authors can create their own XML modules, leaving the XHTML standard unscathed.

• It allows "hybrid" documents in which several DTDs are used in combination. For instance, in theory, it allows web documents to have SVG (Scalable Vector Graphics) modules or MathML modules mixed in with the XHTML content, though the details of making this work have yet to be figured out as of the time of this writing.

The Modularization of XHTML Recommendation was initially released in April 2001. A Second Edition of the Recommendation was introduced as a Working Draft in February 2004.

8.3.1.3. XHTML Basic

The XHTML Basic Recommendation (released in December 2000) is a stripped-down version of modularized XHTML. It is a subset of XHTML elements that are appropriate to such mobile web clients as cell phones, handheld devices, and other information appliances. The definition of a standard, yet extensible, set of XHTML modules for developers of mobile applications and clients allows this document type to be shared across those development communities. It gives them a common starting point. See www.w3.org/TR/xhtml-basic/ for more information.

8.3.1.4. XHTML 1.1

The XHTML 1.1 Recommendation, released in 2001, is a reformulation of XHTML 1.0 (Strict) using the XHTML modules. It is also the first markup language to be fully liberated from legacy functionality of HTML by completely eliminating the elements and attributes that control presentation. Authors are required to put all style and layout information in Cascading Style Sheets.

Some examples of modules in XHTML 1.1 include structure, text, hypertext, lists, object, image, forms, tables, objects, and image maps.

As of this writing, few browsers support an XHTML 1.1 document when it is properly identified as an XML application media type. For this reason, although XHTML 1.1 is the most recent Recommendation, most professional developers use XHTML 1.0 because it can be labeled as HTML text. XHTML media types are discussed further in Chapter 9.

8.3.1.5. XHTML-Print

This document specifies a simple XHTML-based data stream suitable for printing in environments where it is not feasible or desirable to install a printer-specific driver and where some variability in the formatting of the output is acceptable. It is designed to print basic content without regards to layout or presentation. This Recommendation is in development.

8.3.1.6. XHTML 2.0

XHTML 2.0 is a markup language intended for rich, portable web-based applications. It is not intended to be backward compatible with its earlier versions. As of this writing, XHTML 2.0 is in development as a Working Draft.

8.3.2. Three Flavors of HTML 4.01 and XHTML 1.0

Although the W3C has ideas on how HTML should work, they are also aware that it is going to be a while before old browsers are phased out and web authors begin to mark up documents properly. For that reason, both the HTML 4.01 and XHTML 1.0 Recommendations encompass three slightly different specification documents: one "Strict," one "Transitional," and one just for framed documents. These documents, called Document Type Definitions (or DTDs), define every element, attribute, and entity along with the rules for their use. The XHTML DTDs are written following the rules and conventions of XML (Extensible Markup Language), while the HTML DTDs follow SGML syntax. See Chapter 7 for more on XML. The browser uses the DTD to "decode" the markup and check it for validity.

Strict DTD

This version excludes all deprecated elements and attributes (such as font and align) to reinforce the separation of document structure from presentation. Ideally, documents should be tagged strictly for meaning and structure, leaving all presentation to be handled by style sheets.

Transitional DTD

The Transitional DTD includes all deprecated elements and attributes in order to be backwards compatible with the legacy behavior of most browsers. Deprecated elements and attributes are permitted but discouraged from use. This DTD provides a way to ease web authors out of their current habits and toward abiding by standards . Many web authors today choose to use the Transitional DTD while the industry waits for current browsers to offer perfect and consistent CSS support and for older browsers to fade away.

Frameset DTD

The Frameset DTD includes the same elements as the Transitional DTD, with the addition of elements for creating framed web pages (frameset, frame, and noframe). The Frameset DTD is kept separate because the structure of a framed document (where frameset replaces body) is fundamentally different from regular HTML documents. Frames are discussed in Chapter 14.

It is important to specify which version you are using in your document using a DOCTYPE declaration, as modern browsers can use this information to turn on "strict" standards-compliant formatting (Standards Mode), as opposed to the "quirky" behavior of older, nonstandard HTML (Quirks Mode). Of course, if you do specify the DTD, you must stick to it exactly so that your document will be valid (in other words, don't break any rules defined by the DTD). DOCTYPE declarations and switching are discussed further in Chapter 9.