If you write XHTML, and don't follow these two non-XML rules (and indeed the other 14), then it'll probably break when you give it to an HTML parser or browser:
C.2 Empty Elements: Include a space before the trailing / and > of empty elements, e.g. <br />, <hr /> and <img src="karen.jpg" alt="Karen" />. Also, use the minimized tag syntax for empty elements, e.g. <br />, as the alternative syntax <br></br> allowed by XML gives uncertain results in many existing user agents.
C.3. Element Minimization and Empty Element Content: Given an empty instance of an element whose content model is not EMPTY (for example, an empty title or paragraph) do not use the minimized form (e.g. use <p> </p> and not <p />).