This well-formed and valid XHTML document has a self-closing div (
<div />) with a red background above this paragraph. According to the rules of XML, this should be equivalent to
<div></div> and therefore you shouldn't see it. However, when treated like HTML, the
/ in the self-closing tag is treated as a markup error and is simply ignored, so what the browser sees is equivalent to
<div>. As a result, the rest of the document is seen as being inside the div, and the browser thinks the page is a mess.