Centering and Middling a block

by Wentz Wu

HTML

<div class="container">
  <div id="box">
  
  </div>
</div>

CSS

.container {
  position: relative;
  width: 200px;
  height: 200px;
  border: 1px solid grey;
}

#box {
  width: 100px;
  height: 100px;
  border: 1px solid red;
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  margin: auto auto;
}