JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div {
    margin:20px;
    height: 200px;
    width: 200px;
    background-color: #c1c1c1; 
    border:#000 solid 2px;
    position:relative;
}
div:after{
    content:'';
    width:20px;
    height:20px;
    background:#fff;
    position:absolute;
    -moz-transform:rotate(45deg);
    -webkit-transform:rotate(45deg);
    transform:rotate(45deg);
    top:-11px;
    left:10px;
    border-right:#000 solid 2px;
    border-bottom:#000 solid 2px;
}

JavaScript

//http://stackoverflow.com/questions/12619707/create-an-arrow-in-css3