SOURCE

<div class="container">
	<div class="dialog">
        <div class="content">弹框内容</div>
    </dialog>
</div>

.container {
    position: fixed;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    background-color: rgba(0, 0, 0, .5);
    text-align: center;
    font-size: 0;
    white-space: nowrap;
    overflow: auto;
}

.container:after {
    content: '';
    display: inline-block;
    height: 100%;
    vertical-align: middle;
}

.dialog {
    display: inline-block;
    vertical-align: middle;
    text-align: left;
    font-size: 14px;
    white-space: normal;
}

.content {
    width: 300px;
    height: 200px;
    background: #ffffff;
    overflow: auto;
}