
<p>SGML gives the writer of a DTD the ability to exclude specific
elements from being contained within an element. Such
prohibitions (called "exclusions") are not possible in XML.</p>

<p>For example, the HTML 4.0 Strict DTD forbids the nesting of an
'<code>a</code>' element within another '<code>a</code>' element
to any descendant depth. It is not possible to spell out such
prohibitions in XML. Even though these prohibitions cannot be
defined in the DTD, certain elements should not be nested. A
summary of