JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
    <div></div>
</div>

CSS

body{background-color:green}
.parent {
    margin:20px;
    height: 200px;
    width: 200px;
    background-color: #c1c1c1; 
    border:#000 solid 2px;
    position:relative;
}
.parent div{
    position:absolute;
    width:30px;
    height:20px;
    top:-2px;
    left:10px;
    overflow:hidden;
}
.parent div:after{
    content:'';
    width:20px;
    height:20px;
    background:#fff;
    position:absolute;
    -moz-transform:rotate(45deg);
    -webkit-transform:rotate(45deg);
    transform:rotate(45deg);
    border-right:#000 solid 2px;
    border-bottom:#000 solid 2px;
    top:-10px;
    left:5px;
}