half border bottom

1div {
2    width: 500px;
3    height: 100px;
4    position: relative;
5    padding-top: 20px;
6    margin-top: 50px;
7}
8
9div::before {
10    content: '';
11    display: block;
12    position: absolute;
13    top: 0;
14    width: 50%;
15    left: 25%;
16    border-top: 1px solid red;
17}