JavaScript阻止表单提交

<!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>
    <title></title>
    <script type="text/javascript" defer="defer">
        document.getElementById("form1").onsubmit = function (e) {
            alert("Submit prevented");
             e.preventDefault();
            //return false;
        }
    </script>
</head>
<body>
<label>aaa</label>
<form   >
 <input type="submit" value="Submit" />
</form>
</body>
</html>

e.preventDefault()和return false;都可以阻止提交，之前的语句可以验证form里内容是否合法