visibility与display的不同？
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visibility隐藏后他占的位置还在
display隐藏后它在页面上的位置没有了
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visibility
Sets or retrieves whether the content of the object is displayed.
Remarks
Unlike display:none, objects that are not visible still reserve the same physical space in the content layout as they would if they were visible. You can change the visibility through scripting to show and hide overlapping content based on user interaction. For document style scripting information, see Introduction to Dynamic Styles.
As of Microsoft® Internet Explorer 5, a child object can be visible when its parent is hidden. With Internet Explorer 4.0, the parent object must be visible for a child object to be visible.
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visibility 可视 （可视/隐藏）隐藏仍然占用物理空间
display 显示 （块显示/不显示）不显示时不占用物理空间，由相邻元素取代其物理空间
具体取代规则依据html规范而定