In SGML-based HTML 4 certain elements were permitted to omit
the end tag; with the elements that followed implying closure.
This omission is not permitted in XML-based XHTML. All elements
other than those declared in the DTD as EMPTY must
have an end tag.
INCORRECT: unterminated elements
<p>here is a paragraph.<p>here is another paragraph.
CORRECT: terminated elements
<p>here is a paragraph.</p><p>here is another paragraph.</p>