css offset border

1* {
2  box-sizing: border-box;
3}
4
5.container {
6  height: 200px;
7  width: 200px;
8  position: relative;
9  background-color: red;
10  margin: 20px;
11}
12
13.inner {
14  width: 100%;
15  height: 100%;
16  position: absolute;
17  border: 2px solid black;
18  top: -10px;
19  left: -10px;
20  z-index: 10;
21}