JSFiddle - React, Tailwind, and code Playground

HTML

<!-- Erste Kopfzeile -->
  <header class="top-header">
    Erste Kopfzeile
  </header>

  <!-- Zweite Kopfzeile -->
  <header class="nav-header">

    <div class="logo">
      Navigation
    </div>

    <!-- Desktop Navigation -->
    <nav class="nav-links">
      <a href="#">Link1</a>
      <a href="#">Link2</a>
      <a href="#">Link3</a>
    </nav>

    <!-- Hamburger -->
    <div class="hamburger" id="hamburger">
      <span></span>
      <span></span>
      <span></span>
    </div>

  </header>

  <!-- Mobile Menü -->
  <nav class="mobile-menu" id="mobileMenu">
    <a href="#">Link1</a>
    <a href="#">Link2</a>
    <a href="#">Link3</a>
  </nav>

  <!-- Inhalt -->
  <main class="content">

    <div class="grid">

      <div class="box">Box 1</div>
      <div class="box">Box 2</div>
      <div class="box">Box 3</div>
      <div class="box">Box 4</div>
      <div class="box">Box 5</div>
      <div class="box">Box 6</div>
      <div class="box">Box 7</div>
      <div class="box">Box 8</div>
      <div class="box">Box 9</div>
      <div class="box">Box 10</div>

    </div>

  </main>

  <!-- Footer -->
  <footer>
    Fußzeile (immer sichtbar)
  </footer>

<script>
  const hamburger = document.getElementById('hamburger');
  const mobileMenu = document.getElementById('mobileMenu');

  hamburger.addEventListener('click', () => {
    mobileMenu.classList.toggle('active');
  });
</script>

CSS

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }

  body {
    font-family: Arial, sans-serif;
    background: #f1f5f9;
    overflow: auto;
  }

  /* =========================
     ERSTE KOPFZEILE
  ========================== */
  .top-header {
    position: fixed;
    top: 0;
    left: 0;

    width: 100%;
    height: 50px;

    background: #0f172a;
    color: white;

    display: flex;
    align-items: center;
    justify-content: center;

    z-index: 1000;
  }

  /* =========================
     ZWEITE KOPFZEILE
  ========================== */
  .nav-header {
    position: fixed;
    top: 50px;
    left: 0;

    width: 100%;
    height: 60px;

    background: #2563eb;
    color: white;

    display: flex;
    align-items: center;
    justify-content: space-between;

    padding: 0 20px;

    z-index: 1000;
  }

  /* Desktop Links */
  .nav-links {
    display: flex;
    gap: 25px;
  }

  .nav-links a {
    color: white;
    text-decoration: none;
    font-weight: bold;
  }

  .nav-links a:hover {
    text-decoration: underline;
  }

  /* Hamburger */
  .hamburger {
    display: none;
    flex-direction: column;
    cursor: pointer;
    gap: 5px;
  }

  .hamburger span {
    width: 28px;
    height: 3px;
    background: white;
    border-radius: 2px;
  }

  /* Mobile Menü */
  .mobile-menu {
    position: fixed;
    top: 110px;
    left: 0;

    width: 100%;

    background: #1d4ed8;

    display: none;
    flex-direction: column;

    z-index: 999;
  }

  .mobile-menu a {
    color: white;
    text-decoration: none;
    padding: 15px 20px;
    border-top: 1px solid rgba(255,255,255,0.2);
  }

  .mobile-menu a:hover {
    background: rgba(255,255,255,0.1);
  }

  .mobile-menu.active {
    display: flex;
  }

  /* =========================
     FOOTER
  ========================== */
  footer {
    position: fixed;
   ...