CSS hace que el borde gire alrededor del elemento
.b-border{
display: inline-block;
position: relative;
border: solid #333;
border-width: 1px 1px 0px 1px;
padding: 20px;
margin-bottom: 100px;
}
.b-border.border-right{
border-width: 1px 0 1px 1px;
}
.b-border.border-right:after{
content: "";
position: absolute;
right: -30px;
border-top: 1px solid #333;
border-left: none medium;
top: -1px;
left: auto;
width: 30px;
height: 100%;
background: linear-gradient(to top left, white calc(50% - 1px), #000 1px, white 50%);
}
.b-border:after{
content: "";
position: absolute;
left: -1px;
bottom: -15%;
border-left: 1px solid #333;
height: 15%;
width: calc(100% + 1px);
background: linear-gradient(to right bottom, white calc(50% - 1px), #000 1px, white 50%);
}
DevLorenzo