html 固定居中

fixed 居定居中

<div style="position:fixed;top: 50%;left: 50%;transform: translate(-50%, -50%);width:500px;height:500px;border:1px solid red;z-index:99;">
        居中显示的内容
    </div>

遮挡层

<div style="position: absolute;margin: 0;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    height: 2200px;
    position: absolute;
    background-color: #000;
    filter: alpha(opacity=70);
    z-index: 98;
    overflow: hidden;
    opacity: 0.7;"></div>