Despite its name, you don't use Extensible Markup Language (XML) to directly create and mark up web documents. Instead, you use XML technology 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 previous chapter in this book. Nor, then, should it surprise you that one of the first languages defined using XML is an XML-ized 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.
 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 1.0. We describe the salient features of XHTML 1.1 in Section 16.4.
Because of HTML's legacy features and oddities, using XML to describe HTML was not an easy job for the W3C. In fact, certain HTML rules, as we'll discuss later, cannot be represented using 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 almost everything presented elsewhere in this book can be applied to both HTML and XHTML. The differences, both good and bad, are detailed 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.