The majority of HTML is completely compatible with XHTML, and this book is devoted to that majority. In this chapter, however, we talk about the minority: where the HTML 4.01 standard and the XHTML DTD differs. If you truly desire to create documents that are both HTML and XHTML compliant, you must heed the various warnings and caveats we outline in the following sections.
The biggest difference—that's Difference with a capital D and that spells difficult—is that writing XHTML documents requires much more discipline and attention to detail than even the most fastidious HTML author ever dreamed necessary. In W3C parlance, that means your documents must be impeccably well formed. Throughout the history of HTML—and in this book—authors have been encouraged to create well-formed documents, but you have to break rank with the HTML standards for your documents to be considered well formed by XML standards.
Nonetheless, your efforts to master XHTML will be rewarded with documents that are well formed and a sense of satisfaction from playing by the new rules. You will truly benefit in the future, too: through XML, your documents will be able to appear in places you never dreamed would exist (mostly good places, we hope).
One requirement of a well-formed XHTML document is that its elements are nested correctly. This isn't any different from the HTML standards: simply close the markup elements in the order in which you opened them. If one element ...