XHTML is HTML in a stricter form. In the initial period when HTML was introduced and used, a lot of badly formed HTML coding practices were common. Though the then browsers were tolerant of such badly formed code, it did not go well with advanced versions and rightly so. In parallel, during the same time, there were advances in a related technology called XML. XML is used to describe content while HTML is used to present content. The two technologies merged to form XHTML. The first version of XHTML was actually HTML 4.01 with XML. By default HTML has actually become to be referred to as XHTML, and as such the two terms are interchangeably used, including those in this book.
An example of a badly formed HTML is shown in Figure 2.20