codice:
function action() {

  var cont = document.getElementById('index_articoli');
  var divs = cont.getElementsByTagName('DIV');
  
  for (i=divs.length-1;i>=0;i--)  {
    element = divs[i];
	dd= divs[i].id;
	d = document.getElementById(dd);
    element_class = divs[i].className;

    if(d.addEventListener) d.addEventListener("mouseover", function() { this.setAttribute('class',element_class+'_down'); }, false); // mozilla & co.
    else if(d.attachEvent) d.attachEvent("onmouseover", function() { this.setAttribute('class',element_class+'_down'); } ); // IE
    
    if(d.addEventListener) d.addEventListener("mouseout", function() { this.setAttribute('class',element_class); }, false); // mozilla & co.
    else if(d.attachEvent) d.attachEvent("onmouseout", function() { this.setAttribute('class',element_class); } ); // IE
  }

}
codice:
<div id="index_articoli">
  <div id="div_1" class="js">Java</div>

  <div id="div_2" class="css">CSS</div>
  <div id="div_3" class="js">Java</div>
</div>