JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block">

	</div>

CSS

.block {
	background-color: red;
	height: 200px;
	width: 300px;
	position: relative;
}

.block::before {
	content: '';
	position: absolute;
	left: 0;
	
	border-top: 30px solid white;

    border-right: 30px solid red;
}

.block:hover {
	border: 1px solid black;
}