JSFiddle - React, Tailwind, and code Playground

HTML

<header>
<p>Mein erster Test</p>  
</header>
<nav>
  <details>
  <summary>☰ Menü</summary>
    <ul>
    <li><a href="/">Startseite (Home)></a></li>
      <li><a href="#">Seite 1></a>
      <li><a href="#">Seite 2></a>
      <li><a href="#">Seite 3></a>
     </ul>
  </details>

</nav>
<main>     
<article>
<h1>Das ist meine erste Seite</h1>
<p> bla bla bla</p>
<h2>Phase 1.</h2>
<p> bla bla bla</p>
</article>
<aside>
<h1>ASIDE</h1>
<p>by the way</p>
</aside>
</main     
<footer>
<a href="#">Datenschutzhinweise</a>
<span style="margin-left: 2rem; margin-right: 2rem;">|</span>
<a href="/impressum.shtml">Impressum</a>
</footer>

CSS

header p {
  flex: 1 0 10em;
  font-size: 2em;
  margin: 0;
  text-align: center;
}

header nav {
  position: relative;
}
header summary { display: none; }

/* Schließt den Internet Explorer aus */
@supports (width: max-content) {
  header summary {
    font: 100 1.5rem sans-serif;
    cursor: pointer;
    padding: 0 0.2em 0 0;
    user-select: none;
    display: block;    /* Für first-letter, und unterdrückt den Marker */
  }
  header summary::first-letter {
    font-weight: 900;
    text-shadow: 0 -1px;
    padding: 0 0.2em;
    border-radius: 0.2em;
    background-color: #6c6;
  }
  header nav ul {
    position: absolute;
  }
}

header nav ul {
  list-style: none;
  border: 1px solid black;
  border-radius: 0.5em;
  padding: 0.5em;
  background-color: white;
  width: max-content;
}

header nav li {
   padding: 0.1em 0.5em;
}
header nav li.newGroup {
  margin-top: 0.3em;
  padding-top: 0.3em;
  border-top: 1px solid black;
}

main {
  display: grid;
  grid-template-columns: 80% 1fr;
}