How to horizontally center an element

You can apply this CSS to the inner <div>:

#inner {
  width: 50%;
  margin: 0 auto;
}
Of course, you don't have to set the width to 50%. Any width less than the containing <div> will work. The margin: 0 auto is what does the actual centering.

If you are targeting Internet Explorer 8 (and later), it might be better to have this instead:

#inner {
  display: table;
  margin: 0 auto;
}
It will make the inner element center horizontally and it works without setting a specific width.

Working example here:

#inner {
  display: table;
  margin: 0 auto;
  border: 1px solid black;
}

#outer {
  border: 1px solid red;
  width:100%
}
<div id="outer">
  <div id="inner">Foo foo</div>
</div>