JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div {
    position: relative;
    height: 100px;
    background: #900;
}

div:after {
    content: "";
    position: absolute;
    bottom: -40px;
    left: 50%;
    margin-left: -20px;
    border: 20px solid transparent;
    border-top-color: #900;
}