关于伪元素::before&::after的z-index问…

<div id="test"></div>

/*情况1：#test不使用z-index*/
#test{width:120px;height: 60px;background:#bcbcbc;position: absolute; left: 200px;/*z-index:0;*/}

/*情况2：#test使用固定定位*/
#test{width:120px;height: 60px;background:#bcbcbc;position: fixed; left: 200px;/*z-index:0;*/}

/*情况3：#test使用z-index*/
#test{width:120px;height: 60px;background:#bcbcbc;position: absolute; left: 200px;z-index:0;}

#test::before{
    content:'';
    display: block;
    width:60px;height: 60px;background: #333;
    position: absolute;
    left:-20px;
    top:5px;
    z-index: -1;
}
#test::after{
    content:'';
    display: block;
    width:60px;height: 60px;background: #333;
    position: absolute;
    right:-20px;
    top:5px;
    z-index: -1;
}