JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

body{
    background: #ccc;
}
div{
    position: relative;
    width: 100px;
    height: 200px;
    background: #999;
}
div:before{
    content:'';
    position: absolute;
    top: 0;
    left: 0;
    border-style: solid;
    border-width: 20px 20px 0 0;
    border-color: #ccc transparent transparent transparent;
}