JSFiddle - React, Tailwind, and code Playground

HTML

<nav>   <!-- statt <div id="menu"> -->
   <ul>
      <li><a tabindex="0" aria-current="page">Home</a></li>
      <li><a href="./geodaetischeBerechnungen.html">Geodätische Berechnungen</a></li>
      <li><a href="./kontakt.html">Kontakt</a></li>
      <li><a href="./impressum.html">Impressum</a></li>
   </ul>
</nav>

CSS

@font-face {
  font-family: Moontype;
  src: url(Moontype-Bold.woff);
}

body {
    box-shadow: 0 0 2px rgba(0, 0, 0, 0.06);
    color: #545454;
    font-family: Moontype, sans-serif;
    font-size: 16px;
    line-height: 1.5;
    margin: 0 auto;      
    max-width: 800px;
    padding: 2em 2em 4em;
    background: white;
}

nav {
    background-color: #0000ff;
    max-width: 800px;
    padding: 1% 1% 1% 2%;
}
nav ul {
    display: flex;
    justify-content: space-between;
    list-style-type: none; padding: 0 2em;
    font-family: Moontype, sans-serif;
    color: #ffffff;
    font-size: 20px;
}
nav a {
    color: #ffffff;
    font-size: 20px;
    text-decoration:none;
    outline-color:white;
}
nav a[aria-current] {
    border-bottom: solid 5px #ffffff;
}