Because you now have to follow the rules when writing XHTML, it's important that you spend some time understanding what those rules are. In some cases, following a rule means just a simple behavior change; in others, it seems like you are working with a vastly different language.
Listing 2.1 shows some HTML code that is not XHTML compliant. Indeed, the code is technically not even compliant with the syntax rules put forward in the HTML standard. In spite of this, a browser still renders it with no problem (see Figure 2.1).
<HTML> <HEAD> <TITLE>OnlinePizza.Com Order Form</TITLE> ...