$("#frmSignin").validate({
    debug: false,
    errorClass: "authError",
    errorElement: "span",
    rules: {
        username: {
            required: true,
            minlength: 10
        },
        password: {
            required: true
        }
    },
    messages: {
        username: {
            required: "Please enter your username"
        },
        password: {
            required: "Please enter your password"
        }
    }
});

label.error

span.authError {
    // Your error element style.
}

$("#frmSignin").validate({
    debug: false,
    errorClass: "authError",
    errorElement: "span",
    rules: {
        username: {
            required: true,
            minlength: 10
        },
        password: {
            required: true
        }
    },
    messages: {
        username: {
            required: "Please enter your username"
        },
        password: {
            required: "Please enter your password"
        }
    },
    highlight: function(element, errorClass) {
        $(element).removeClass(errorClass);
    }
});