Vertical center a div inside another div using flex-box

Center align a div not just horizontally but also vertically using display flex.

HTML

<div class="wrapper">
    <div class="container">
        Centering a div using "<strong>display: flex</strong>"
    </div>
</div>

CSS

.wrapper {
  display: flex;
  justify-content: center;
  width: 400px;
  height: 300px;
  border: 1px solid #555;
}

.container {
  align-self: center;
  padding: 20px;
  border: 1px solid #cd0000;
}