Since the last restructuring of htmlblock/phpblock invalid xhtml is produced when the config options htmlok/phpok are turned off.
See the example on
http://wiki.splitbrain.org/wiki:syntax:
<HTML><p style="border:2px dashed red;">And this is some block HTML</p></HTML>
produces (simplified without the geshi stuff)
<p>
<pre><p style="border:2px dashed red;">And this is some block HTML</p></pre>
</p>
<pre> inside <p> is not allowed (neither in strict, nor transitional).