Now for the good news. It turns out that XHTML is very similar to "pure" HTML and, if you've followed the HTML 4.0 guidelines, you will probably have to undertake a minimal number of changes to make the switch to XHTML. The W3C has supplied a comparison between HTML 4.0 and XHTML to aid in this transition. Typical modifications will probably include the following:
Use of lowercase HTML tags throughout, due to XML's case sensitivity. This means that elements such as <TABLE> or <BR> should be converted to <table> and <br>, respectively.
Non-empty elements must have end tags. To specify a new paragraph, many HTML developers simply use the <p> tag without a closing </p>. Like all XML documents, HTML requires that all elements include a beginning and ending tag and that they also be properly nested.
Empty elements must have an end tag, or the start tag must end with />. This means that line breaks such as <br> must now be coded as <br></br> or <br />. The latter example (<br />) will not be accepted by some HTML browsers, making the longer <br></br> syntax necessary.
Later in this article I'll mention a few tools to aid your transition. Before discussing those, however, let's look at a valid XHTML document.