1. z-index
z-index 属性用于设置元素的叠放层次，属性值可以为一个整数，整数值越大越优先显示
注意：z-index只对开启了定位的元素有效
示例如下：
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title>div {width: 200px;height: 200px;}.box1 {background-color: #c7edcc;position: relative;left: 50px;top: 100px;z-index: -2;}.box2 {background-color: #fde6e0;position: absolute;left: 150px;top: 150px;z-index: -1;}.box3 {background-color: #dce2f1;float: left;}.box4 {background-color: red;position: fixed;left: 125px;top: 225px;}</head><body><div class="box1">相对定位</div><div class="box2">绝对定位</div><div class="box3">浮动</div><div class="box4">固定定位</div></body></html>