Web Design in a Nutshell, 3rd Edition

Empty Elements

This need for termination extends to empty elements as well. So instead of just inserting a line break as <br>, XHTML requires the element to be terminated. You can simply add a slash before the closing bracket, indicating the element’s ending. So in XHTML, a line break is entered as <br/>.

The notion of closing empty elements can cause some browsers (namely Netscape 4) to complain, and even new browsers to have problems when content is sent as text/html, so to keep your XHTML digestible to those browsers, be sure to add a space before the closing slash (<br />). This allows the closed empty tag to slide right through. See Table 8-1 for a complete list of empty elements.