JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

.triangle {
   width: 100px;
    height: 100px;
    position: relative;
    overflow: hidden;
    box-shadow: 0 16px 10px -17px rgba(0, 0, 0, 0.5);
}
.triangle:after {
   content: "";
    position: absolute;
    width: 50px;
    height: 50px;
    background: #999;
    -webkit-transform: rotate(45deg);
    top: 75px;
    left: 25px;
    box-shadow: -1px -1px 10px -2px rgba(0, 0, 0, 0.5);
}