상위 Div 내에서 Div를 수직 중앙에 배치하는 방법은 무엇입니까?

<div class="container">
  <div class="inner">
    ...
  </div>
</div>

.container {
  display: table-cell;
  vertical-align: middle;
  height: 500px;
  width: 500px;
}

.inner {
  display: inline-block;
  width: 200px;
  height: 200px;
}

.container {
  position: relative;
  height: 500px;
  width: 500px;
}

.inner {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 200px;
  height: 200px;
}

.container {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 500px;
  width: 500px;
}

.inner {
  width: 200px;
  height: 200px;
}