<div class="container">
<div class="box blueBox"></div>
</div>
.container { width: 100%; position: absolute; top: 50%; margin-top: -50px; left: 0; }
.blueBox { width: 100px; height: 100px; margin: 0 auto; background: blue; border-radius: 50%; }
<div class="container">
<div class="box blueBox"></div>
</div>
.container { width: 100%; position: absolute; top: 50%; margin-top: -50px; left: 0; }
.blueBox { width: 100px; height: 100px; margin: 0 auto; background: blue; border-radius: 50%; }