JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
        <ul>
            <li>1</li>
            <li>2</li>
            <li>3</li>
            <li>4</li>
        </ul>
        <div class="box"></div>
</div>

CSS

.wrapper{
position:relative;
}

ul{
position:relative;
top:1px;
margin:0px;
list-style:none;
}

li{
display:inline-block;
width:100px;
height:30px;
border:1px solid #ccc;
border-bottom:none;
border-radius:5px 5px 0px 0px;
background:#fff;
}

.box{
display:block;
width:600px;
height:400px;
border:1px solid #ccc;
}