Do you use the target=blank attribute on links? If yes, you might know that XHTML 1.0 Strict don’t allow it. A good solution to this problem should be using JQuery to make links opening in new windows:
$('a[@rel$='external']').click(function(){
this.target = "_blank";
});
Usage:
<a href="http://www.whileifblog.com/" rel="external">Whileifblog</a>
