JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  
    <div class="corner">
    
    </div>
  
  </div>

CSS

.container {
  margin: auto;
  width: 400px;
  height: 400px;
  background-color: black;
  outline: 1px solid #333333;
  position: relative;
  overflow: hidden;
}

.corner {
    position: absolute;
    bottom: -68px;
    right: -66px;
    width: 86px;
    height: 82px;
    background-color: #F1F2F3;

    -webkit-transform: rotate(45deg);
    -moz-transform: rotate(45deg);
    -ms-transform: rotate(45deg);
    -o-transform: rotate(45deg);
    transform: translateZ(1px) rotate(45deg);
    
    -webkit-backface-visibility: hidden;
    -moz-backface-visibility: hidden;
    -ms-backface-visibility: hidden;
    -o-backface-visibility: hidden;
    backface-visibility: hidden;
    border-style: solid;
    border-width: 14px 16px 28px;
    
    -moz-border-image: url(http://s24.postimg.org/aq0pokg41/curve_border_grey.png) 14 16 28 repeat;
    -webkit-border-image: url(http://s24.postimg.org/aq0pokg41/curve_border_grey.png) 14 16 28 repeat;
    -o-border-image: url(http://s24.postimg.org/aq0pokg41/curve_border_grey.png) 14 16 28 repeat;
    border-image: url(http://s24.postimg.org/aq0pokg41/curve_border_grey.png) 14 16 28 fill repeat;
    
    -moz-background-clip: padding;
    -webkit-background-clip: padding;
    background-clip: padding-box;
}