JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

body {
    background:blue;
}

div {
    width:200px;
    height:100px;
    background:black;
    margin:15px;
    position:relative;  
}
div:before {
    content:"";
    position:absolute;
    top:0;
    left:0;
    width: 0; 
    height: 0; 
    border-top: 100px solid white; 
    border-right: 100px solid transparent; 
}