如图，一开始元素的透明度是30，鼠标移上的时候，透明度慢慢增加，到透明度100停止。鼠标移出，透明度慢慢减少，减少到30.
要点一：因为无法直接获取和改变透明度的值，可以把透明度值赋给一个变量，让变量变化，最后把变量的值再赋给元素的透明值。
var alpha=30;
要点二：判断目标值和目前透明值，来判定是正向速度还是负向速度。
if(target > alpha){
speed = 2;
}else{
speed = -2;
}
要点三：如果透明值达到目标值，关掉定时器，否则透明值继续变化。最后把值赋给元素，因为透明度有兼容问题,所以要写上两个写法。
if(alpha == target){
clearInterval(timer);
}
else{
alpha = alpha + speed;
run.style.filter = 'alpha(opacity='+alpha+')';
run.style.opacity = alpha/100;
document.title = alpha;
}
最后，上代码
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
body{margin:0; padding:0;}
#run{width:100px; height:100px; background:#06c; position:absolute; border:1px solid #000; opacity:0.3; filter:alpha(opacity=30);}
</style>
<script>
window.onload = function(){
var run = document.getElementById("run");
var btn = document.getElementById("btn");
var speed = 1;
var timer = null;
var alpha=30;
run.onmouseover = function(){
startrun(100);
}
run.onmouseout = function(){
startrun(30);
}
function startrun(target){
clearInterval(timer);
timer = setInterval(function(){
if(target > alpha){
speed = 2;
}else{
speed = -2;
}
if(alpha == target){
clearInterval(timer);
}
else{
alpha = alpha + speed;
run.style.filter = 'alpha(opacity='+alpha+')';
run.style.opacity = alpha/100;
document.title = alpha;
}
},30)
}
}
</script>
</head>
<body>
<div id="run"></div>
</body>
</html>