走马灯原理——代码很简单，异常很神奇，哪位高手来挑挑错？

走马灯原理——代码很简单，错误很神奇，谁来挑挑错？？

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>走马灯原理</title>
    <style type="text/css">
        #container
        {
            position:relative;
            margin:0 auto;
            width:200px;
            height:200px;
            background:#0000cc;
        }
        #child
        {
            position:absolute;
            top:200px;
            left:0;
            width:200px;
            height:220px;
            background:#FF0000;
        }
        #btnControl
        {
            position:absolute;
            left:100px;
            top:100px;
            width:80px;
            height:50px;
        }
    </style>
    <script type="text/jscript">
        function step(){
            var child=document.getElementById("child");
            var new_top=child.style.top.match(/([0-9,.\-]+)px/)[1]-5;
            child.style.top=new_top+"px";

        }
    </script>
</head>
<body>
    <div id="container">
        <div id="child"></div>
    </div>
    <input type="button" id="btnControl" value="滚动" onclick="step()" />
</body>
</html>

三个错误：

1、——————————————————————————————————————

2、——————————————————————————————————————

3、——————————————————————————————————————

走马灯原理——代码很简单，异常很神奇，哪位高手来挑挑错？

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