JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box"></div>

CSS

#box{
height:40px;
width:100px;
background:red;
}

#box:hover{background:blue;}

#box:after{
content: "";
height: 10px;
position: absolute;
width: 0;
margin-top:40px;
margin-left:40px;
border: 10px solid transparent;
border-top-color: red;
}