JSFiddle - React, Tailwind, and code Playground
HTML
<div>
</div>CSS
div { position:relative; background-color:#333; padding:20px; margin:20px; float:left; }
div:before { content:""; display:block; padding:5px; background-color:#f60; border:2px solid white; position: absolute; top:-2px; right:-2px; border-bottom-left-radius: 10px; }