JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

.main {
        width: 300px;
        height: 200px;
        background-color: blue;
    }
    .cornered {
        width: 260px;
        height: 0px;
        border-top: 40px solid blue;
        border-right: 40px solid white;
    }