JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li><a href="#" class="active">item 1</a></li>
    <li><a href="#">item 2</a></li>
    <li><a href="#">item 3</a></li>
</ul>

CSS

ul{
    margin: 0;
    padding: 0;       
    list-style: none;
    overflow: hidden;
    margin-bottom: 5px;
    padding-bottom: 5px;
    border-bottom: 1px solid green; 
}

ul li{
    float: left;   
    margin-right: 10px;
}

ul li a{
    text-decoration: none;
    display: block;
}

ul li a.active{
    border-bottom: 5px solid red;    
}