要点一：
if(list_li.length>=1){
list.insertbefore(li,list_li[0]);
}else{
list.appendchild(li);
}
从在前面插入新内容，如果没有新内容，就是在后面插入新内容。
要点二：
var height=li.offsetheight;
li.style.height='0';
取得li的高度，然后再li的高度设置为0,因为高度的变化是从0到现有高度。
要点三：
startrun(li,height,height,function(){
startrun(li,opacity,100);
})
先是高度变化，再是透明度变化
最后，上代码：
<!doctype html>
<html>
<head>
<meta charset=gb2312 />
<title>无标题文档</title>
<style>
<!--
body,ul,li{margin:0; padding:0; font:12px/1.5 arial;}
#list{width:400px; margin:10px auto;}
#list li{list-style:none; padding:5px 0 ;
overflow:hidden; border-bottom:1px dotted #ccc;
filter:alpha(opacity:0); opacity:0; vertical-align:middle;}
-->
</style>
<script>
<!--
window.onload = function(){
var btn = document.getelementbyid(btn);
var con = document.getelementbyid(con);
var list = document.getelementbyid(list);
var list_li = list.getelementsbytagname(li);
btn.onclick = function(){
var li = document.createelement(li);
li.innerhtml = con.value;
con.value='';
if(list_li.length>=1){
list.insertbefore(li,list_li[0]);
}else{
list.appendchild(li);
}
var height=li.offsetheight;
li.style.height='0';
startrun(li,height,height,function(){
startrun(li,opacity,100);
})
}
}
function getstyle(obj,name){
if(obj.currentstyle){
return obj.currentstyle[name];
}else{
return getcomputedstyle(obj,false)[name];
}
}
function startrun(obj,attr,target,fn){
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);
if(fn){
fn();
}
}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)
}
//-->
</script>
</head>
<body>
<textarea id=con cols=50 rows=5></textarea>
<input id=btn name= type=button value=发布>
<ul id=list>
</ul>
</body>
</html>