清除浮动

这个例子的 HTML 是这样的：(div-1 > (div-2 + div-3)) + div-4

div-1

div-2

div-3

div-4

div-1 float left

overflow: hidden

float: none

float: left
div-2 float right

clear: none

clear: left

clear: right

clear: both
div-3 float left

clear: none

clear: left

clear: right

clear: both
div-4 float left

clear: none

clear: left

clear: right

clear: both