float 属性最初只用于在成块的文本内浮动图像(文字环绕), 后来也用在多列布局上
姑且定为兼容旧浏览器的布局技术
flexbox 请
https://developer.mozilla.org/zh-CN/docs/Learn/CSS/CSS_layout/Floats
The end of the clearfix hack?
https://rachelandrew.co.uk/archives/2017/01/24/the-end-of-the-clearfix-hack/