JSFiddle - React, Tailwind, and code Playground

HTML

<ul id = "nav">
    <li>Accueil</li>
    <li>Recherche</li>
    <li>Contact</li>
</ul>

CSS

#nav {
    width: 100%;
    text-align: justify;
    margin: 0 0 3em 0;
    padding: 8px;
    list-style: none;
    border-bottom: 1px solid #ccc; 
    border-top: 1px solid #ccc;
}

#nav:after {
    content: '';
    display: inline-block;
    width: 100%;
}

#nav li {
    width: 10%;
    display: inline-block;
    padding: 4px 2%;
    text-align: center;
    text-decoration: none;
    background-color: #f2f2f2;
    border: 1px solid #ccc;
    margin-right:12%;
}
#nav:before  {
    width: 15%;
    display: inline-block;
    padding: 4px 15px;
    text-align: center;
    text-decoration: none;
    content:''
}