<style type="text/css">
    form label.hide {
        display: none;
    }
</style>
<script type="text/javascript">
    function sendForm( self ) {
        var hasSend = true;
        for( var i = 0; i < self.length; i++ ) {
            if ( self[i].value ) {
                self[i].parentNode.className = "hide";
            } else {
                self[i].parentNode.className = "";
                self[i].focus();
                hasSend = false;
                break;
            }
        }
        return hasSend;
    }
</script>
<form action="" method="post" onsubmit="return sendForm( this );">
    <label>Login: <input type="text" name="login" /></label>
    <label class="hide">Password: <input type="password" name="password" /></label>
</form>