<div class="parent">
    <div class="child"></div>
</div>

.parent{
    width:400px;
    height:400px;
    background:#afa;
    position:relative;
}
.child{
    position:absolute;
    left:0;
    bottom:0;
    right:0;
    top:0;
    margin:auto;
    background:yellow;
    width:100px;
    height:100px;
}

.parent{
    width:400px;
    height:400px;
    background:#afa;
    position:relative;
}
.child{
    position:absolute;
    left:50%;
    top:50%;
    transform:translate(-50%,-50%);
    background:yellow;
    width:100px;
    height:100px;
}

.parent{
    width:400px;
    height:400px;
    display:flex;
    justify-content:center;
    align-items:center;
    background:#afa;
}
.child{
    width:100px;
    height:100px;
    background:yellow;
}

.parent{
    display:table-cell;
    width:400px;
    height:400px;
    background:#afa;
    text-align:center;
    vertical-align: middle;
}
.child{
    display:inline-block;
    width:100px;
    height:100px;
    background:yellow;
}