JSFiddle - React, Tailwind, and code Playground

by poonia

HTML

<div id="corner"></div>
<div id="corner-shadow"></div>

CSS

#corner {
    position: absolute;
    top: 100px;
    left: 100px;
    height: 0px;
    width: 0px;
    border: 20px solid blue;
    border-color: transparent;
    border-top-color: blue;
    border-top-style: solid;
    border-top-width: 25px;
    border-right-width: 20px;
    border-left-width: 20px;
    border-bottom-width: 0px;
}
#corner-shadow {
    position: absolute;
    top: 100px;
    left: 116px;
    height: 20px;
    width: 8px;
    background: red;
    z-index: -1;
    box-shadow: -4px 5px 30px rgba(0,0,0,1),4px 5px 30px rgba(0,0,0,1);
}