流式布局

float:left, right, none(不允许进行流动)
clear:both, right, left,none

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clear:none // 允许两边都可以有浮动对象
clear:left // 不允许左边有浮动对象
clear:right // 不允许右边有浮动对象
clear:both // 不允许两边有浮动对象

clear means an element won't bubble up beside a previous floating element.

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