<style type="text/css">
        .div1 {
            width: 100px;
            height: 100px;
            border: 1px solid #000000;
            position: relative;
        }

        .div2 {
            width: 40px;
            height: 40px;
            background-color: red;
            position: absolute;
            margin: auto;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
        }
    </style>

    <div class="div1">
        <div class="div2 ">
        </div>
    </div>

<style type="text/css">
        .div1 {
            width: 100px;
            height: 100px;
            border: 1px solid #000000;
            display: table-cell;
            vertical-align: middle;
        }

        .div2 {
            width: 40px;
            height: 40px;
            background-color: red;
            margin: auto;
        }

    </style>

    <div class="div1">
        <div class="div2">
        </div>
    </div>

<style type="text/css">
    .div1 {
        width: 100px;
        height: 100px;
        border: 1px solid #000000;
        display: flex;
        /*!*flex-direction: column;*!可写可不写*/
        justify-content: center;
        align-items: center;
    }

    .div2 {
        height: 40px;
        width: 40px;
        background-color: red;
    }
</style>
<div class="div1 ">
    <div class="div2 ">
    </div>
</div>

<style type="text/css">
    .div1 {
        width: 100px;
        height: 100px;
        border: 1px solid #000000;
        position: relative;
    }

    .div2 {
        height: 40px;
        width: 40px;
        background-color: red;
        position: absolute;
        top: 50%;
        left: 50%;
        -ms-transform: translate(-50%, -50%);
        -moz-transform: translate(-50%, -50%);
        -o-transform: translate(-50%, -50%);
        transform: translate(-50%, -50%);
    }
</style>
<div class="div1 ">
    <div class="div2 ">
    </div>
</div>