JSFiddle - React, Tailwind, and code Playground

HTML

<nav>
    <a href="#" id="open" onclick="">Menü öffnen</a>
    <a href="#" id="close" onclick="">Menü schließen</a>
    <ul>
        <li><a href="#">Startseite</a></li>
        <li><a href="#">Moderator</a></li>
        <li><a href="#">Archiv</a></li>
        <li><a href="#">Impressum</a></li>
    </ul>
</nav>

CSS

nav a {
    background: #efefef;
    color: #000;
    cursor: pointer;
    display: block;
    margin-bottom: 1px;
    padding: 1em;
    text-decoration: none;
}
nav a:active, nav a:hover {
    background: #999;
}
#open:focus {
    left: -9999px;
    position: absolute;
}
#close {
    display: none;
}
#open:focus + #close {
    display: block;
}
nav > ul {
    left: calc(-100% - 1em);
    list-style: none;
    margin: 0;
    padding: 0;
    position: relative;
}
#open:focus ~ ul {
    left: 0%;
    transition: left .2s linear;
}