Could you also give the exact HTML code that you set in the description ?
Looking at the code of the page, I would say that it comes from the description itself.
One option might be to change the line:
$description = strip_tags(JHTML::_('content.prepare',$optionInfo->product_description));
to:
$description = $this->escape(strip_tags(JHTML::_('content.prepare',$optionInfo->product_description)));
in the file "option" of the view "product" via the menu Display->views, so you might want to try that.