CodePen Embed - margin-inline: auto;

<div class="wrapper">
  <div class="box"></div>
</div>

.wrapper {
  width: 100%;
  height: 250px;
  padding: 100px 0;
  background: #eee;
}

.box {
  width: 250px;
  height: 250px;
  background: #9fcf00;
  margin-inline: auto;
}

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