<div class="wrapper">
<div class="content">
</div>
</div>
.wrapper{
height :400px;
border: solid 1px #ff0000;
position: relative;
}
.content{
position: absolute;
border: solid 1px blue;
left: 0;
right: 0;
top: 0;
bottom: 0;
margin:auto;
width: 20px;
height: 20px;
}