JavaScript实现表单提交后禁用提交按钮

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title>表单提交后按钮禁用</title>
<script type="text/javascript">
function sub() {
  document.forms[0].elements[0].disabled = true;
  document.forms[0].submit();
}
</script>
</head>
<body>
  <form action="login.action" method="post">
    <input type="button" name="btn" value="提交表单"
 onclick="return sub();" />
  </form>
</body>
</html>