#slider {
    position: relative;
    width: 1200px;
    margin: auto;
    display: flex;
    align-items: center;
}

#precedent,
#suivant {
    cursor: pointer;
    opacity: 1;
    position: absolute;
    font-size: 50px;
    color: #FFF;
    background-color: rgba(0, 0, 0, 0.3);
    padding: 10px;
}

#precedent {
    left: 0;
}

#suivant {
    right: 0;
}

#slider:hover #precedent,
#slider:hover #suivant {
    opacity: 1;
}