@keyframes alfa {
    from {opacity: 0;}
    to {opacity: 1;}
}

#superContenedor {
    margin: 0 auto;
    overflow: hidden;
    width: 90%;
}

#contenedor {
    overflow-y: auto;
    padding-right: 25px;
    width: 100%;
}

#contenido {
    animation-name: alfa;
    animation-duration: 2.5s;
}