表单提交验证

<form action ="..." methot ="post"><input type ="text" name ="username" id="username"><input type ="password" name ="password" id ="password"><input type ="submit" value ="提交" onclick ="return valid();"> </form>JavaScript代码；；<script type="text/javascript">   function valid(){     var name = document.getElementById("username").value;     var pass = document.getElementById("password").value;     if(name==""){        alert("用户名不能为空");        return false;     }     if(pass==""){        alert("密码不能为空");        return false;     }     return true;   }</script>