css horizontal center

.parent {
  display: flex;
  justify-content: center;
  align-items: center;
}

#inner {  
	margin: auto 0;
}

#outer {
  width:100%;
  display: flex;
  justify-content: center;
}


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

#inner {
  width: 50%;
  margin: 0 auto;
}