CodePen Embed - position: absoluteで上下中央にする

<div style="position: relaitve;">
<div class="kuromaru" style="width: 50px; height:50px; background-color: #000; border-radius: 50%;"></div>
</div>

.kuromaru{
  position: absolute;
  top: 0;
  bottom: 0;
  margin: auto;
}

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