PmWiki markup [[<<]] is rendered as <br clear=all />
which is not XHTML1 strict.
Perhaps it could be changed to
## [[<<]] (break)
Markup('[[<<]]','inline','/\\[\\[<<\\]\\]/',"<br class='clearall' />");
and class clearall added to $HTMLStylesFmt['markup']
as
.clearall { clear:both; }
or defined as <br style='clear:both' />
~Hans