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HTML 

              
                

<nav>
  <button class="menubutton" onclick="this.classList.toggle('showmenu')">Menü</button> 
  
  <ul>
    <li class="current"><a href="#">Startseite</a></li>
    <li><a href="#">News</a></li>
    <li><a href="#">Über&nbsp;uns</a></li>
    <li><a href="#">Kontakt</a></li>
  </ul>
</nav>

<hr>

<p>Zum Testen können Sie das JavaScript für die mobile Navigation per Klick aus- und einschalten und die Fensterbreite verändern. </p>
<button class="jstoggle off" onclick="document.documentElement.classList.remove('js'); document.documentElement.classList.add('no-js')">Mobile Navigation ohne JavaScript</button> &nbsp; &nbsp;
<button class="jstoggle on" onclick="document.documentElement.classList.remove('no-js'); document.documentElement.classList.add('js')">Mobile Navigation mit JavaScript</button> 

  <h2>Über diese Navigation</h2> 

<p>Diese Navigation ist responsiv und folgt dem Prinzip des <em>progressive enhancement</em>: </p>

  <ol> 
    <li>Ohne JavaScript   
      <ul> 
        <li>wird die Navigationsliste offen angezeigt
und der Menübutton versteckt. </li> 
        <li>können Besucher können auch ohne
JavaScript navigieren. </li> 
      </ul> 
    </li> 
    <li>Mit JavaScript  
      <ul> 
        <li>ist der
Menübutton sichtbar und die Navigationsliste ausgeblendet. </li> 
        <li> wird die
Navigationsliste durch Antippen des Menübuttons sanft eingeblendet.</li> 
      </ul>
    </li> 
  </ol>
  <p>Weitere Infos <a href="https://html-und-css.de/responsive-navigation-progressive-enhancement/" target="_blank" rel="noopen noreferrer">siehe Beitrag auf der Website zum Buch</a>.
!

CSS 

              
                body {
  font-family: system-ui, -apple-system, "Segoe UI", Helvetica, Arial, sans-serif;
  line-height: 1.5; 
}

/* Styling for the navigation */ 

/* basic styling - mobile first */
nav { margin-top: 2rem; }
nav ul { 
  display: flex; 
  flex-flow: column; 
  list-style: none; 
  padding: 0; 
  margin: 0; 
}

nav a {
  display: block; 
  text-decoration: none; 
  background: #333; color: white; 
  padding: 0.5rem 1rem;
  /* pull right margin over the padding, for .current, :hover and :focus */
  margin-right: -1rem;   
}

nav .current a, 
nav a:hover, 
nav a:focus { 
  background: #0b74b8; 
  color: white;   
}

.no-js .menubutton { 
  display: none; 
}

/* style the menubutton */
.js .menubutton { 
  display: flex; 
  align-items: center; 
  cursor: pointer; 
  
  background: #333; 
  color: white; 
  font: inherit; 
  
  padding: 0.5rem 1rem; 
  border: 0; 
  margin: 0; 
}

/* Three lines = https://unicode-table.com/en/2630/ (instead of SVG icon) */ 
.js .menubutton::before {
  content: "\2630\00A0"; 
  /* manually adjust vertical alignment, not necessary with SVG icon */
  margin-bottom: 3px; 
}

/* hide navigqtion list on mobile */
.js nav ul {
  /* max-height can be animated with transition, see below */
  max-height: 0; 
  overflow: hidden; 
  padding: 0; 
} 

/* show navigation list after click/tap on menubutton */
.js .menubutton.showmenu + ul {
  /* value should be big enough to contain the menu in all circumstances */ 
  max-height: 100rem; 
  transition: max-height 500ms; 
}
/* X icon = https://unicode-table.com/en/2716/ */ 
.js .menubutton.showmenu::before {
  content: "\2716\00A0"; 
  margin-bottom: 0;
}

/* style horizontal navigation   */

@media screen and (min-width: 600px) {

  /* Hide menubutton */ 
  .menubutton {
    display: none !important; 
  } 
  
  nav { 
    max-width: none; 
  }
  
  /* list items horizontal */ 
  nav ul { 
    max-height: none !important; 
    flex-flow: row; 
    padding: 0; 
  }

  nav li {
    flex: 1; 
    max-width: 10rem; 
    text-align: center; 
    border-left: 1px solid #eee; 
    margin: 0; 
  }
  nav li:first-child {
    margin: 0; 
  }
  nav li:last-child {
    border-right: 1px solid #eee; 
    margin: 0; 
  }
  nav a {
    margin-right: 0; 
  }

} /* Ende @media */ 


/* Styling for the rest of the Pen */ 

hr {
  margin: 3rem ; 
}

.jstoggle {
  cursor: pointer; 
  font: inherit; 
  background-color: gainsboro; 
  color: #333; 
  padding: 0.5rem 1rem; 
  border: 0; 
  margin: 0 0 1rem 0; 
}

.js .jstoggle.on, 
.no-js .jstoggle.off,
.jstoggle:hover, 
.jstoggle:focus {
  background: darkred; 
  color: white; 
}

.js .jstoggle.on::after { content: ": AN"; font-weight: bold; }
.js .jstoggle.off::after { content: ""; }
.no-js .jstoggle.on::after { content: ""; }
.no-js .jstoggle.off::after { content: ": AN"; font-weight: bold; }

ol ul li::marker {
  content: "... "; 
}
!

JS
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