效果如下，添加新内容，会高度先变化，然后再渐显出来。
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>