求帮忙 无法运行
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<script type="text/javascript">
function showMes(){
alert("hello world");
}
var btn1=document.getElementById("btn1");
btn1.attachEvent("onclick",showMes);
</script>
</head>
<body>
<input type="button" id="btn1" value="确定" name="按钮1">
</body>
</html>