JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container tabs">
    <ul>
        <li>Option 1</li>
        <li class="active">Option 2</li>
    </ul>
</div>

CSS

body {
    background: #e1e1e1;
}
.container {
    width: 500px;
}
.container ul {
    float: left;
    background: #fff;
}
.container li {
    padding: 1em;
    list-style-type: none;
    float: left;
    position: relative;
}
.container ul li.active {
    
}
.container ul li.active:after {
    content: " ";
    border-left: 10px solid transparent;
    border-right: 10px solid transparent;
    border-top:  10px solid #fff;
    width: 0;
    height: 0;
    position: absolute;
    left: 40%;
    top: 100%;
}