JSFiddle - React, Tailwind, and code Playground

HTML

<html>
    <head/>
    
    <body>
        <nav id="navigation">
      <ul id="navi_main">
        <li class="Startseite"><a href="index.html">Startseite</a>
        </li>
        <li class="Inhalt"><a href="content.html">Inhalt</a>
          <ul class="dropdown_navi">
            <li><a href="#">Linkziel 1</a></li>
            <li><a href="#">Linkziel 2</a></li>
            <li><a href="#">Linkziel 3</a></li>
          </ul>
        </li>
        <li class="Angebot"><a href="offer.html">Angebot</a>
          <ul class="dropdown_navi">
            <li><a href="#">Linkziel 1</a></li>
              <li id="d1" class="test"><a href="#">Linkziel 1.1</a></li>
              <li id="d2" class="test"><a href="#">Linkziel 1.2</a></li>
            <li><a href="#">Linkziel 2</a></li>
              <li id="d3" class="test"><a href="#">Linkziel 2.1</a></li>
              <li id="d4" class="test"><a href="#">Linkziel 2.2</a></li>
              <li id="d5" class="test"><a href="#">Linkziel 2.3</a></li>
            <li><a href="#">Linkziel 3</a></li>
              <li id="d6" class="test"><a href="#">Linkziel 3.1</a></li>
              <li id="d7" class="test"><a href="#">Linkziel 3.2</a></li>
          </ul>
        </li>
        <li class="about"><a href="about.html">Über mich</a>
        </li>
        <li class="kontakt"><a href="contact.html">Kontakt</a>
        </li>
      </ul>
    </nav>
        
        
    </body>
    
    
</html>

CSS

ul{
  list-style-type: none;
}

ul#navi_main{
  text-align: center;
  font-weight: bold;
}

ul#navi_main a{
  text-decoration: none;
  color: black;
}

ul#navi_main a:hover{
  text-decoration: none;
    background:red;
}

ul#navi_main li{
  float: left;
  position: relative;
  list-style-type: none;
  margin-top: 15px;
  margin-left: 10px;
  line-height: 30px;
  -webkit-transition: all 200ms ease-in;
  -moz-transition: all 200ms ease-in;
  -ms-transition: all 200ms ease-in;
  -o-transition: all 200ms ease-in;
  transition: all 200ms ease-in;
}

ul#navi_main li.startseite{
  width: 100px;
  height: 30px;
}

ul#navi_main li.inhalt{
  width: 100px;
  height: 30px;
}

ul#navi_main li.angebot{
  width: 100px;
  height: 30px;
}

ul#navi_main li.about{
  width: 100px;
  height: 30px;
}

ul#navi_main li.kontakt{
  width: 100px;
  height: 30px;
}

ul#navi_main li.startseite:hover{
  width: 100px;
  height: 30px;
  background: #BFBFBF;
  border-radius: 10px;
  box-shadow: 3px 3px 3px #000000;
  -webkit-transition: all 200ms ease-in;
  -moz-transition: all 200ms ease-in;
  -ms-transition: all 200ms ease-in;
  -o-transition: all 200ms ease-in;
  transition: all 200ms ease-in;
}

ul#navi_main li.inhalt:hover{
  width: 100px;
  height: 30px;
  background: #BFBFBF;
  border-top-left-radius: 10px;
  border-top-right-radius: 10px;
  box-shadow: 3px 3px 3px #000000;
}

ul#navi_main li.angebot:hover{
  width: 100px;
  height: 30px;
  background: #BFBFBF;
  border-top-left-radius: 10px;
  border-top-right-radius: 10px;
  box-shadow: 3px 3px 3px #000000;
}

ul#navi_main li.about:hover{
  width: 100px;
  height: 30px;
  background: #BFBFBF;
  border-radius: 10px;
  box-shadow: 3px 3px 3px #000000;
}

ul#navi_main li.kontakt:hover{
  width: 100px;
  height: 30px;
  background: #BFBFBF;
  border-radius: 10px;
  box-shadow: 3px 3px 3px #000000;
}

ul#navi_main li ul.dropdown_navi{
  background: #BFBFBF;
  position: absolute;
  left: 0;
  top: 30px;
  width: 150px;
  padding: 0px;
 ...