<div class="thinner-border"></div>
.thinner-border {
position: relative;
height: 100px;
/* border: 1px solid red; */
/* background: red; */
}
.thinner-border:before {
content: '';
position: absolute;
width: 200%;
height: 200%;
border: 1px solid #000;
transform-origin: 0 0;
transform: scale(0.5, 0.5);
box-sizing: border-box;
}