SOURCE

<div class="a"></div>
<div class="b"></div>

        .a {
            width: 100px;
            height: 100px;
            margin: 10px;
            border: 1px solid red;

        }

        .b {
            width: 200px;
            height: 200px;
            margin: 50%;
            /* margin: 20px;; */
            border: 1px solid blue;
        }