.nav { width: 100%; height: 80px; @include box-shadow(0 1px 7px); position: relative; font-family: RalewayL; font-size: 2em; display: flex; justify-content: center; align-items: center; }

.nav li { list-style: none; text-align: center; display: inline-block; padding: 17px 20px; }
.nav li:hover { list-style: none; text-align: center; display: inline-block; padding: 7px 20px; border-bottom: 2px solid black; margin-top:-5px;  transition: all 0.3s linear;}

.nav a {  text-decoration: none; color: black; }
        
 <nav class="nav">   
    <a href="index.php"><li>Accueil</li></a>
    <a href="menus.php"><li>Menus</li></a>
    <a href="carte.php"><li>La Carte</li></a>
    <a href="latelierduchef.php"><li>l'Atelier du chef</li></a>
    <a href="contact.php"><li>Contact</li></a>
</nav>