这里可实现5个元素中随便一个，鼠标移上去透明度渐渐增加，鼠标移出，透明度渐渐减小的效果。
要点一：
var speed = 0; if(target>obj.alpha){ speed = 5; }else{ speed = -5; }
根据目标值和当时值的对比，来决定是正向还是负向速度。
要点二：
for(i=0; i<runs_li.length; i++){ runs_li[i].timer = null; runs_li[i].alpha = 30; runs_li[i].onmouseover = function(){ startrun(this,100); } runs_li[i].onmouseout = function(){ startrun(this,30); } }
给每一个元素加上各自的透明度值，各自的透明度变化分开。
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,ul,li{margin:0; padding:0;} #runs{width:300px; margin:10px auto;} #runs li{width:80px; height:80px; background:#06c; list-style:none; float:left; margin:10px; display:inline; filter:alpha(opacity=30); opacity:0.3;} </style> <script> window.onload = function(){ var runs = document.getElementById("runs"); var runs_li = runs.getElementsByTagName("li"); var i=0; for(i=0; i<runs_li.length; i++){ runs_li[i].timer = null; runs_li[i].alpha = 30; runs_li[i].onmouseover = function(){ startrun(this,100); } runs_li[i].onmouseout = function(){ startrun(this,30); } } } function startrun(obj,target){ clearInterval(obj.timer); obj.timer = setInterval(function(){ var speed = 0; if(target>obj.alpha){ speed = 5; }else{ speed = -5; } if(obj.alpha == target){ clearInterval(obj.timer); }else{ obj.alpha = obj.alpha + speed; obj.style.filter = "alpha(opacity="+obj.alpha+")"; obj.style.opacity = obj.alpha/100; } },30) } </script> </head> <body> <ul> <li>1</li> <li>2</li> <li>3</li> <li>4</li> <li>5</li> </ul> </body> </html>