overflow: hidden which is explained in the W3C content will be cut, and the rest is not visible
So why can clear float it?
A: set overflow: hidden elements to be cut height based on its content and hide, then when the height of the element is not given, we must first calculate the height, if only the parent element and the floating element subclasses, then the floating element involved in the calculation, the sub-element calculations on passing reaching the goal of cleaning up floating
Only understand fur, then dig a little deep down, do not know ...