为什么我的无法关闭新窗口
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>close()</title>
<script type="text/javascript">
var mywin;
function openUrl()
{
var mywin=window.open("http://www.imooc.com");
}
function closeUrl()
{
mywin.close();
}
</script>
</head>
<body>
<input name="Obutton" type="button" onClick="openUrl()" value="open" />
<br>
</br>
<input name="Cbutton" type="button" onClick="closeUrl()" value="close" />
</body>
</html>