SOURCE

<div class="wrapper">
    <div class="content"></div>
</div>

.wrapper{
    height :400px;
    border: 1px solid red;
    position: relative;
}

.content{
    width: 100px;
    height: 100px;
    border: 1px solid #0000ff;
    background-color: aqua;

    position: absolute;
    left: 50%;
    top: 50%;
    margin-left: -50px;
    margin-top: -50px;
   
}