1

.wraper {
  position: relative;
  .box {
    position: absolute;
    top: 50%;
    left: 50%;
    width: 100px;
    height: 100px;
    margin: -50px 0 0 -50px;
  }
}

2

.wraper {
  position: relative;
  .box {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
  }
}

3

.wraper {
  .box {
    display: flex;
    justify-content:center;
    align-items: center;
    height: 100px;
  }
}

4

.wraper {
  display: table;
  .box {
    display: table-cell;
    vertical-align: middle;
  }
}

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