JSFiddle - React, Tailwind, and code Playground

HTML

<div class="cornered"></div>
<div class="main">Hello</div>

CSS

.cornered {
    width: 160px;
    height: 0px;
    border-bottom: 40px solid red;
    border-right: 40px solid white;
}
.main {
    width: 200px;
    height: 200px;
    background-color: red;
}