:after和:before生成的内容具有内联元素的属性，在不设置display：block的情况下设置宽高无效
:after和:before生成的内容具有absolute的性质，默认都是以父元素的左上角为原点
:after和:before生成的内容和父内容的位置关系为：父元素<:before=:after，也就是说父元素位置最下，:after和:before位置同等，同时设置position：absolute的情况下，可通过设置z-index来设置上下关系
:after，:before
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