JSFiddle - React, Tailwind, and code Playground

HTML

<div id="menu">
  <ul>
    <li>Menu1
      <ul class="submenu">
        <li>Menu1Sub1</li>
        <li>Menu1Sub2</li>
        <li>Menu1Sub3</li>
        <li>Menu1Sub4</li>
      </ul>
    </li>
    <li>Menu2
      <ul class="submenu">
        <li>Menu2Sub1</li>
        <li>Menu2Sub2</li>
        <li>Menu2Sub3</li>
      </ul>
    </li>
  </ul>
</div>

CSS

#menu { 
    position: relative;
    background-color: black;
        text-decoration:none;
}
#menu li { 
        background-color: green;
    padding: 10px;
    height: 30px;
        list-style: none;
        line-height:30px; 
        float:left;
}
#menu li > ul { 
        display: none; 
}
#menu > ul > li {
    border: 2px solid white;
}
#menu > ul > li:hover {
    border: 2px solid black;
}

#menu li:hover > ul { 
        display: block;
    position: absolute;
    top: 54px;
    left: 0;
} 
#menu li ul li {
        float:left;
        position:relative;
}