这样写为什么不可以？
<!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>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>我们互动下</title>
<script type="text/javascript">
var ainput=document.getElementsByTagName("input")[0];
ainput.onclick=function pop(){
alert("关注JS高级篇");
}
</script>
</head>
<body>
<input type="button" value="点击我"/>
</body>
</html>
