how to center a div

div {
    width: 100px;
    height: 100px;
    background-color: red;
    
    position: absolute;
    top:0;
    bottom: 0;
    left: 0;
    right: 0;
    
    margin: auto;
}

<div id="outer">
  <div id="inner">Foo foo</div>
</div>