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 cols="50" rows="5"></textarea> <input type="button" value="发布"> <ul> </ul> </body> </html>