为什么点按钮的时候出错呢？
程序代码：
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Untitled Document</title>
</head>
<script>
function ff(){
var oDiv = document.getElementById("div1");
oDiv.style.removeProperty("background-color");
}
</script>
<body>
<div id="div1" style="background-color:red;height:50px;width:50px" onmouseover="this.style.backgroundColor='blue'">
</div>
<input type="button" value="use Methods" onclick="ff();"/>
</html>
