SOURCE

<div class="father">
    <div class="son">

    </div>
</div>

/* .father{
    overflow: hidden;
    height: 100px;
    width: 100px;
    background-color: royalblue
}

.son{
    height: 30px;
    width: 30px;
    background-color: hotpink;
    margin: 0 auto;
    margin-top: 35px
} */


/* .father{
    height: 100px;
    width: 100px;
    background-color: royalblue;
    position: relative
}

.son{
    height: 30px;
    width: 30px;
    background-color: hotpink;
    position: absolute;
    left: 50%;
    top: 50%;  
    margin-left: -15px;
    margin-top: -15px;
} */

.father{
    height: 100px;
    width: 100px;
    background-color: royalblue;
    position: relative
}

.son{
    height: 30px;
    width: 30px;
    background-color: hotpink;
    position: absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;  
    margin: auto;
}