Web Design in a Nutshell, 3rd Edition

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.