div {
margin: auto;
position: absolute;
top: 0;
left: 0;
right: 0
bottom: 0;
width: 100px;
height: 100px;
border:1px solid red;
}
将产生一个div， width: 100px; height: 100px; 外边框 1px 红色实线 ；
div居于屏幕中间，对于居于中间这点，我无法理解,为何下面的做法不可以？
div {
margin: auto;
position: absolute;
top: 0;
left: 0;
width: 100px;
height: 100px;
border:1px solid red;
}
回复讨论(解决方案)
问题1
top: 0;
left: 0;
为何不可以代替
top: 0;
left: 0;
right: 0
bottom: 0;
问题2
margin:auto为何不可以代替
margin: auto;
top: 0;
left: 0;
right: 0
bottom: 0;
<html> <head> <style type="text/css"> div { margin: auto; position: absolute; right: 0; bottom: 0; top: 0; left: 0; width: 100px; height: 100px; border:1px solid red; } </style> </head> <body> <div></div> </body></html> 打开浏览器，改变浏览器的视区的大小，发现依然居中
因为该div设置了position为absolute，而且设置了置入值（left\right\top\bottom），根据元素包含块的定义，因为div的父级没有position为“absolute”、“relative”或者“fixed”的祖先元素，所以该div的包含块为初始包含块，即根元素html的包含块，我在控制台选中html标签的时候，可以看出整个视区都被选中，所以这个时候初始包含块就是视区
根据css2.1中‘top’ + ‘margin-top’ + ‘border-top-width’ + ‘padding-top’ + ‘height’ + ‘padding-bottom’ + ‘border-bottom-width’ + ‘margin-bottom’ + ‘bottom’ = 包含块的高度
设置margin的属性为auto恰好填充了空白
可以在控制台看出随着视区大小变化，margin的值随之变化
问题1：
如果只有left属性和top属性 那么margin属性为auto没有起到任何的作用
只有设置了left和right，或者top与bottom的时候，margin为auto的时候才会去填充左右或者上下的空白，达到left属性0，right属性为0，top属性为0，bottom属性为0 的效果，不然div只有100*100那么大，不可能达到left为0又right为0的效果啊！只有添加margin，增大在文档流的占用空间
问题2：
同理问题1，不设置置入值（left\right\top\bottom），这个时候margin属性为auto没有起到任何的作用
<html> <head> <style type="text/css"> div { margin: auto; position: absolute; right: 0; bottom: 0; top: 0; left: 0; width: 100px; height: 100px; border:1px solid red; } </style> </head> <body> <div></div> </body></html> 打开浏览器，改变浏览器的视区的大小，发现依然居中
因为该div设置了position为absolute，而且设置了置入值（left\right\top\bottom），根据元素包含块的定义，因为div的父级没有position为“absolute”、“relative”或者“fixed”的祖先元素，所以该div的包含块为初始包含块，即根元素html的包含块，我在控制台选中html标签的时候，可以看出整个视区都被选中，所以这个时候初始包含块就是视区
根据css2.1中‘top’ + ‘margin-top’ + ‘border-top-width’ + ‘padding-top’ + ‘height’ + ‘padding-bottom’ + ‘border-bottom-width’ + ‘margin-bottom’ + ‘bottom’ = 包含块的高度
设置margin的属性为auto恰好填充了空白
可以在控制台看出随着视区大小变化，margin的值随之变化
问题1：
如果只有left属性和top属性 那么margin属性为auto没有起到任何的作用
只有设置了left和right，或者top与bottom的时候，margin为auto的时候才会去填充左右或者上下的空白，达到left属性0，right属性为0，top属性为0，bottom属性为0 的效果，不然div只有100*100那么大，不可能达到left为0又right为0的效果啊！只有添加margin，增大在文档流的占用空间
问题2：
同理问题1，不设置置入值（left\right\top\bottom），这个时候margin属性为auto没有起到任何的作用
上面描述增大在文档流的占用空间不正确，position属性为absolute已经脱离了文档流，应该是增大该div的占用空间比较准确一点
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