[Deff]

<div id="navigation">
		<ul>
                     <li><a href="#">п1</a></li>
                     <li><a href="#">п2</a></li>
                     <li><a href="#">п3</a></li>
                     <li><a href="#">п4</a></li>
		</ul>
</div>
<style type="text/css">
a.this {
 color:red;
}
</style>
<script type="text/javascript">
var a =  document.getElementById("navigation");
var b =  a.getElementsByTagName("a");
var THref = location.href;
leng = b.length
for(var i=0; i<leng; i++){
  if(b[i].href==THref){alert("A");b[i].className='this';}

}
</script>

Заменил класс navigation - на id