JSFiddle - React, Tailwind, and code Playground

HTML

<div class="tri-top-right">
 
</div>

CSS

.tri-top-right:before {
    content:'';
    display: block;    
    border-top: 500px solid black;
    border-left: 500px solid transparent;
    position: absolute;
    top: 0;
    right: 0;
}