SOURCE

window.onload=function() {
	var oDiv=document.getElementsByTagName('div');
    for(var i=0; i<oDiv.length; i++) {
    	oDiv[i].timer=null;
    	oDiv[i].alpha=30;
    	oDiv[i].onmouseover=function() {
    		startMove(this,100);
    	}
    	oDiv[i].onmouseout=function() {
    		startMove(this,30);
    	}
    }
}
  // var alpha=30;
function startMove(obj,iTarget) {
  clearInterval(obj.timer);
  obj.timer=setInterval(function() {
  var speed=(iTarget-obj.alpha)/10;
  speed=speed>0?Math.ceil(speed):Math.floor(speed);
  if(obj.alpha==iTarget) 
  {
    clearInterval(obj.timer);
  }
  else
  {
   	obj.alpha+=speed;
    obj.style.filter="alpha(opacity:"+obj.alpha+")";//对字符串的理解不牢靠.也就是字符串和数值的相加
    //其实字符串和数值相连接的时候，我们只要全部用双引号，或者使用单引号，就可以看的十分清楚了。
   	obj.style.opacity=obj.alpha/100;
   	document.title=obj.style.opacity;
  }
    },30);
}

<div id="div1"></div>
<div id="div2"></div>
<div id="div3"></div>

div {width: 200px; height: 200px; background: red; float: left; margin: 10px; filter: alpha(opacity: 30); opacity: 0.3;}