HTML & XHTML: The Definitive Guide, 6th Edition

Chapter 16. XHTML

Despite its name, you don’t use the Extensible Markup Language (XML) to directly create and mark up web documents. Instead, you use XML to define a new markup language, which you then use to mark up web documents. This should come as no surprise to anyone who has read the preceding chapter in this book. Nor, then, should it surprise you that one of the first languages defined using XML is an XMLized version of HTML, the most popular markup language ever. HTML is being disciplined and cleaned up by XML, to bring it back into line with the larger family of markup languages. This standard is XHTML 1.0.^[*]

Because of HTML’s legacy features and oddities, using XML to describe HTML was not an easy job for the World Wide Web Consortium (W3C). In fact, certain HTML rules, as we’ll discuss later, cannot be expressed with XML. Nonetheless, if the W3C has its way, XHTML will ultimately replace the HTML we currently know and love.

So much of XHTML is identical to HTML’s current standard, version 4.01, that you can apply almost everything presented elsewhere in this book to both HTML and XHTML. We detail the differences, both good and bad, in this chapter. To become fluent in XHTML, you’ll first need to absorb the rest of this book, and then adjust your thinking to embrace what we present in this chapter.

^[*] Throughout this chapter, we use “XHTML” to mean the XHTML 1.0 standard. There is a nascent XHTML 1.1 standard that diverges from HTML 4.01 and is more restrictive than XHTML ...