SOURCE

<div class="outside">
    <div class="inside"></div>
</div>

div.outside {
    background-color: skyblue;
    width: 350px;
    height: 300px;

    position: relative;
}

div.inside {
    background-color: lightcoral;
    width: 100px;
    height: 100px;

    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    /* 或者 */
    /* margin-top: -50px; */
    /* margin-left: -50px; */
}