JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="main">
      <li><a href="link.php">A dropdown</a>
        <ul class="dropdown">
           <li><a href="link2.php">A submenu</a></li>
        </ul>
      </li>
    </ul>

CSS

#main li:hover a {
      border-bottom: 1px solid black;
    }

.dropdown {
    display: none;
}

#main li:hover ul {
    display:block;
}