Floating, floating clear, :: after, :: before,: hover

Floating, floating clear

https://www.jianshu.com/p/c23700db2597

Pseudo-elements (double colon): behind the last child element of a newly added sub-element does not exist in the document flow

https://www.jianshu.com/p/831b600d1c46

Pseudo-class: Adding in its own body

https://www.cnblogs.com/Yfling/p/7259949.html