HTML Comes of Age: XHTML - Extensible HTML (Page 3 of 7 )
The XHTML recommendation was published by the W3C on 26 January 2000, and refers to XHTML as "a bridge to the future." According to various versions of the W3C specification, XHTML offers three major advantages to Web site developers: extensibility, portability, and modularity. XHTML is extensible by adding new elements without altering the entire DTD (document type definition) that the document is based on.
With all the hype about the extensibility of XHTML, I was confused at first that the spec doesn't have much information in it about how to define your own tags. That's because XHTML isn't there yet. It is 'merely' a reformulation of HTML 4.01 in XML, so that you create a Web page in XML with references to one of three DTDs that I'll discuss below. The current XHTML recommendation is the first step in realizing the extensible dream of HTML.
The second major advantage is portability, sometimes referred to as interoperability. Most Internet access is through browsers on desktop computers, though more and different types of devices are constantly being introduced. Some of these devices, such as cell phones and household appliances, won't have the processing power of a desktop computer, and browsers on them will be less tolerant of malformed markup to render the document. XHTML is designed to make Web documents accessible and interoperable across platforms, in part by enforcing a rigorous coding standard.
Modularity made it into the specification late in the process, and will be fleshed out in XHTML 1.1. It acknowledges the growing role that the Web is playing in handheld devices. Browsers on these devices will not need all XHTML elements, so XHTML allows subsets of elements. This way the new language of the Web will be scalable both up and down, a critical feature for its success on the Web and on new wireless devices.