为什么打开的是两个窗口，关闭只能关一个
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>close()</title>
<script type="text/javascript">
var mywin=window.open("http://www.imooc.com");
//mywin.close();
function a_close(){
mywin.close();
}
</script>
</head>
<body>
<input type="button" onClick='a_close()' value='关闭'/>
</body>
</html>
