<div style="position: relaitve;">
<div class="kuromaru" style="width: 50px; height:50px; background-color: #000; border-radius: 50%;"></div>
</div>
.kuromaru{
  position: absolute;
  top: 50%;;
  left: 50%;
  margin: auto;
  transform: translateX(-50%) translateY(-50%);
}

External CSS

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External JavaScript

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