2-6习题，为何将_blank改为_self时下面语句不起作用？
<!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",'_blank','top=100,width=400,height=200,left=15');
var flag=confirm ("点击确定来关闭这个窗口！");
if (flag==true)
{
mywin.close();
}
</script>
</head>
<body>
</body>
</html>
