Due to the fact that XHTML is an XML application, certain
practices that were perfectly legal in SGML-based HTML 4.0 <a
href="#ref-html4">[HTML]</a> must be changed.</p>

<h2><a name="h-4.1" id="h-4.1">4.1 Documents must be
well-formed</a></h2>

<p><a href="#wellformed">Well-formedness</a> is a new concept
introduced by <a href="#ref-xml">[XML]</a>. Essentially this
means that all elements must either have closing tags or be
written in a special form (as described below), and that all the
elements must nest