Any description element is allowed to have HTML embedded in it.

It isn't XHTML embedded as children.

It is entity-encoded HTML:

<description>&lt;p>I'm a paragraph but you don't know it.&lt;/p></description>

Or even worse:

<description>How many ampersands do you see: &amp;amp;amp; ?</description>

This makes it difficult but not impossible to process with XML tools.