nav {
    width: 800px;
    margin: auto;
}
ul {
    display: flex;              /* en ligne*/
    list-style: none;           /* supprimer le puce*/
    margin: 0;
    padding: 0;
}
li {
    background-color: black;
    padding: 15px;
    text-align: center;
    flex-grow: 1;
    flex-shrink: 0;
    flex-basis: auto;
}
li:nth-child(1), li:nth-child(2), li:nth-child(3) {
    border-right: 1px solid #ccc;
}
li a {
    color: yellow;
    text-decoration: none;      /* supprimer le soulignement du lien*/
    font-family: Arial, Helvetica, sans-serif;
    font-size: 20px;
}

@media screen and (max-width: 800px) {
    nav {
        width: 90%;
    }
    ul {
        flex-direction: column;
    }
    li:nth-child(1), li:nth-child(2), li:nth-child(3) {
        border-right: 0;        /*supprimer bordure*/
        border-bottom: 1px solid #ccc;
    }
}