要点一：
function getstyle(obj,name){
if(obj.currentStyle){
return obj.currentStyle[name];
}else{
return getComputedStyle(obj,false)[name];
}
}
从样式表中根据id和属性名取值。
要点二：
if(attr == "opacity"){
cur = Math.round(parseFloat(getstyle(obj,attr))*100);
}else{
cur = parseInt(getstyle(obj,attr));
}
如果设置的是透明度的值，因为有可能会是小数点，所以需要用parseFloat，然后有可能会有小数，用round方法四舍五入取整。
如果设置的非透明度值，用parseInt，可以只取数值部分
其它的注意点，前面几篇都有提到，此处不再赘述。
最后，上代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>无标题文档</title>
<style>
body,ul,li{margin:0; padding:0;}
#runs li{width:80px; height:80px; background:#06c;
list-style:none; margin:10px; border:1px solid #000;
filter:alpha(opacity=30); opacity:0.3;}
</style>
<script>
window.onload = function(){
var runs = document.getElementById("runs");
var runs_li = runs.getElementsByTagName("li");
runs_li[0].onmouseover = function(){
startrun(this,"width",300);
}
runs_li[0].onmouseout = function(){
startrun(this,"width",80);
}
runs_li[1].onmouseover = function(){
startrun(this,"height",300);
}
runs_li[1].onmouseout = function(){
startrun(this,"height",80);
}
runs_li[2].onmouseover = function(){
startrun(this,"fontSize",50);
}
runs_li[2].onmouseout = function(){
startrun(this,"fontSize",14);
}
runs_li[3].onmouseover = function(){
startrun(this,"opacity",100);
}
runs_li[3].onmouseout = function(){
startrun(this,"opacity",30);
}
}
function startrun(obj,attr,target){
clearInterval(obj.timer);
obj.timer = setInterval(function(){
var cur = 0;
if(attr == "opacity"){
cur = Math.round(parseFloat(getstyle(obj,attr))*100);
}else{
cur = parseInt(getstyle(obj,attr));
}
var speed = (target - cur)/8;
speed = speed>0?Math.ceil(speed):Math.floor(speed);
if(cur == target){
clearInterval(obj.timer);
}else{
if(attr == "opacity"){
obj.style.filter='alpha(opacity='+(cur+speed)+')';
obj.style.opacity=(cur+speed)/100;
}else{
obj.style[attr] = cur+speed+"px";
}
}
},30);
}
function getstyle(obj,name){
if(obj.currentStyle){
return obj.currentStyle[name];
}else{
return getComputedStyle(obj,false)[name];
}
}
</script>
</head>
<body>
<ul id="runs">
<li style="top:0">1</li>
<li style="top:90px;">2</li>
<li style="top:180px;">3</li>
<li style="top:270px;">4</li>
</ul>
</body>
</html>
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