html结构

<div class="outside-container">
    <div class="inner-container">需要水平、垂直居中的DIV</div>
</div>

CSS3样式

.outside-container {
    background: rgba(0,0,0,.5);
    position: fixed;
    z-index: 9999;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    /* 居中内部div样式 */
    display: -webkit-flex;
    -webkit-justify-content: center;
    -webkit-align-items: center;
}

.inner-container {
    position: relative;
    background-color: #FFF;
    padding: 20px;
    border-radius: 4px;
    /* ... */
}

应用场景：弹窗提示、弹层loading等等