原生JavaScript实现Div匀速运动动画

<html> 
<head> 
<meta charset="gb2312"> 
<head>
<title>javascript实现的简单动画</title>
<style type="text/css">
#mydiv
{
 width:50px;
 height:50px;
 background-color:green;
 position:absolute;
}
</style>
<script type="text/javascript"> 
window.onload=function()
{
 var mydiv=document.getElementById("mydiv");
 var start=document.getElementById("start");
 var stopmove=document.getElementById("stopmove");
 var x=0;
 var flag;
 function move()
 {
  x=x+1;
  mydiv.style.left=x+"px";
 }
 start.onclick=function()
 {
  clearInterval(flag);
  flag=setInterval(move,20);
 }
 stopmove.onclick=function()
 {
  clearInterval(flag);
 }
 
}
</script>
<body>
<input type="button" id="start" value="开始运动" />
<input type="button" id="stopmove" value="停止运动" />
<div id="mydiv"></div>
</body>
</html>