SOURCE

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

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

    /* 该属性要写在块级元素里 */
    text-align: center;
}

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

    /* 但是子元素要为行内或行内块元素 */
    display: inline-block;
}