html
<div class="box"></div>
css
.box {
width: 100px;
height: 100px;
border: 1px solid deeppink;
position: absolute;
}
.box::before {
content: '';
width: 80%;
height: 100%;
border-bottom: 1px solid #ddd;
border-top: 1px solid #ddd;
bottom: -1px;
position: absolute;
top: -1px;
left: 10%;
}
.box::after {
content: '';
width: 100%;
height: 80%;
border-left: 1px solid #eee;
border-right: 1px solid #eee;
position: absolute;
left: -1px;
right: -1px;
top: 10%;
}
效果：
