window.onload=drag;

function drag(){

var div1=document.getElementById('div1');

div1.onmouseover=function(){changeOpacity(100);}

div1.onmouseout=function(){changeOpacity(20);}

}

var timer=null;

var alpha=20;

function changeOpacity(num){

var div1=document.getElementById('div1');

clearInterval(timer);

timer=setInterval(function(){

    var speed=0;

if(num>alpha){speed=10;} else{speed=-10;}

if(alpha==num){clearInterval(timer);}else{

alpha+=speed;

div1.style.filter='alpha(opacity:'+alpha+')';

div1.style.opacity=alpha/100;}

},50);

}