<div id="flex1">세로 가운데 정렬</div>
#flex1 {
  border: 1px solid #000;
  height:300px;
  width:200px;
  flex: 1;
  display: -webkit-flex;
  display: flex;
  -webkit-align-items: center;
  align-items: center;
  -webkit-justify-content: center;
  justify-content: center;
}

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