.9">4.9 SGML exclusions</a></h2>

<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 s