Edit in JSFiddle

<div class="one">
  <div class="one1">
    left & right is 0, margin is 0 auto
  </div>
  <div class="one2">
   left & right is not 0, margin is 0 auto
  </div>
</div>

.one{ background-color:#D3F3ED;position:relative; heigit:100%; height:300px; text-align:center;line-height:90px;}

.one1{ background-color:#9EDDD0;position:absolute; width:40%; top:10%; height:30%; margin:0 auto;left:0;right:0;}
.one2{ background-color:#2ED7D2;position:absolute; width:40%; top:50%; height:30%; margin:0 auto;}