XHTML, and its predecessor language HTML, are just markup languages designed for use on the Web. They both use tags that are written using regular printable characters. Indeed, the two languages are nearly identical-- Chapter 2 looks at them, and their differences, in more detail.
To illustrate how these languages work, we will start with a simple example HTML (not XHTML) document, shown in Figure ... omitted content ... example as an XHTML document to help highlight the differences between HTML and XHTML.
The rules for writing correct XHTML and HTML documents are slightly different, and are outlined in more ... omitted content ... treated as HTML.