<div class="square"></div>
.square,
.square:before,
.square:after {
border: solid #000 3px;
}
.square {
position: relative;
width: 100px;
height: 100px;
border-left: 0;
margin: 0 auto;
}
.square:before,
.square:after {
content: '';
display: block;
position: absolute;
border-right: 0;
}
.square:before {
top: 0;
right: 100%;
width: 150%;
height: 40px;
margin: -3px -3px 0 0;
}
.square:after {
left: 0;
top: 40px;
bottom: 0;
}