css浮动位置未定义是啥意思？

最近在看css规范，在浮动这一章节有这一段不是很理解，求大神指教But in CSS 2.1, if, within the block formatting context, there is an in-flow negative vertical margin such that the float's position is above the position it would be at were all such negative margins set to zero, the position of the float is undefined.链接：https://www.w3.org/TR/CSS2/vi...