Edit in JSFiddle

#cover{
    position: absolute;
    top: 0;
    z-index: 1;
    background: navy;
    opacity: 0;
    -webkit-animation: opaco 5s;
    -moz-animation: opaco 5s;
}

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

@-webkit-keyframes opaco{
    from{
        opacity: 1;
    }
    to{
        opacity: 0;
    }
}

@-moz-keyframes opaco{
    from{
        opacity: 1;
    }
    to{
        opacity: 0;
    }
}