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}