JSFiddle - React, Tailwind, and code Playground

HTML

<div><span></span></div>

CSS

div {
    position: relative;
    width: 100px;
    height: 100px;
    margin: 20px;
    border: 1px solid #000;
}
div:before {
    display: block;
    content: "";
    width: 20px;
    height: 20px;
    position: absolute;
    top: -10px;
    left: -10px;
    border-top: 1px solid #000;
    border-left: 1px solid #000;
}
div:after {
    display: block;
    content: "";
    width: 20px;
    height: 20px;
    position: absolute;
    top: -10px;
    right: -10px;
    border-top: 1px solid #000;
    border-right: 1px solid #000;
}
span:before {
    display: block;
    content: "";
    width: 20px;
    height: 20px;
    position: absolute;
    bottom: -10px;
    left: -10px;
    border-bottom: 1px solid #000;
    border-left: 1px solid #000;
}
span:after {
    display: block;
    content: "";
    width: 20px;
    height: 20px;
    position: absolute;
    bottom: -10px;
    right: -10px;
    border-bottom: 1px solid #000;
    border-right: 1px solid #000;
}