JSFiddle - React, Tailwind, and code Playground

HTML

<header>
<ul id="nav">
    <li><a href="">link 1</a></li>
    <li><a href="">link 2</a></li>
    <li><a href="">link 3</a></li>
</ul>
</header>
<div id="content">
    body content
</div>

CSS

header {
    margin: 0;
    padding: 0;
    left: 0;
    top: 0;
    width: 100%;
    position: absolute;
    background: #000000;
    padding-bottom: 20px;
    height: 30px;
    font: 16px normal Calibri, sans-serif;
    text-transform: uppercase;
}
ul {
    list-style-type: none;
    display: block;
    margin: 0 0 0 20px;
    padding: 0;
}
ul li {
    display: inline-block;
    padding: 0;
    margin-right: 5px;
}
ul li a{
    display: block;
    border-bottom: 1px solid #fff;
    font-size: 19px;
    color: white;
    text-decoration: none;
    padding-bottom: 5px;
  /*  transition: border-bottom 0.5s; */
}
ul li a:hover{
    border-bottom: 5px solid #fff;
    background: #333;
    font-size: 19px;
    padding-bottom: 1px;
}

#content {
    margin-top: 35px;
}