JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li><a href="#">1st lvl</a></li>
     <li><a href="#">This black</a>
       <ul class="sub-menu">
           <li><a href="#">When hover me</a></li>
            <li><a href="#">and me</a></li> 
       </ul>
     </li> 
     <li><a href="#">1st lvl</a></li> 
     <li><a href="#">1st lvl</a></li> 
     <li><a href="#">1st lvl</a></li> 
</ul>

CSS

ul li {
    display: inline-block;
}

ul li a {
    position: relative;
    padding: 5px 10px;
}

ul li:hover {
    background-color: #000;
}

.sub-menu {
    position: absolute;
    top: 30px;
    border: solid 1px red;
}

.sub-menu li {
    display: block;
}