border far from element css

1#content{
2  width: 100px;
3  min-height: 100px;
4  margin: 20px auto;
5  background:#000;
6  position:relative;
7}
8#content:after {
9  content: '';
10  position: absolute;
11  left: 0px;
12  right: 0px;
13  bottom: -25px;
14  border: red 3px solid;
15}