secret.css

header,
main,
footer {
    animation: 3s linear 0s 1 normal both paused disappear;
}

#secret {
    animation: 3s linear 0s 1 normal both running appear;
}

@keyframes disappear {
    from {
        opacity: 1;
    }

    to {
        opacity: 0;
    }
}

@keyframes appear {
    from {
        opacity: 0;
    }

    to {
        opacity: 1;
    }
}