<form method="post" name="login" action="logging.php?action=login" >

<form method="post" name="login" action="logging.php?action=login" onSubmit="if(this.username.value.length<1||this.username.value.length>50){alert('請輸入用戶名或UID');return false;}else{if(this.password.value.length<1||this.password.value.length>50){alert('請輸入密碼');return false;}}">