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>

EDIT

With flexbox it is very easy to style the div horizontally and vertically centered.

#inner {  
  border: 0.05em solid black;
}

#outer {
  border: 0.05em solid red;
  width:100%;
  display: flex;
  justify-content: center;
}

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

To align the div vertically centered, use the property align-items: center.

If you don't want to set a fixed width on the inner div you could do something like this:

#outer {
  width: 100%;
  text-align: center;
}

#inner {
  display: inline-block;
}

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

That makes the inner div into an inline element that can be centered with text-align.