JSFiddle - React, Tailwind, and code Playground

HTML

<div class="triangle">
      
    </div>

CSS

.triangle:before {
    border: inset 6px;
    content: "";
    display: block;
    height: 0;
    width: 0;
    border-color: transparent transparent red transparent;
    border-bottom-style: solid;
    position: absolute;
    top: 50px;
    left: 50px;
    z-index: 89;
    box-shadow: 0px 0px 2px 2px #AAA;
}