包含块和绝对定位元素
元素绝对定位时，会完全从文档流删除，然后相对于其包含块定位，其边界根据偏移属性(left,top等)放置。
对于一个非根元素，如果其position为absolute，包含块为最近的position不是static的祖先元素。如果这个祖先元素是块级元素，包含块则设置为该元素的内边距边界；如果这个祖先是行内元素，包含块则设置为该祖先元素的内容边界。
自动边偏移
元素绝对定位时，如果除了bottom外某个偏移属性设置为auto，会有特殊的行为。
原文：
when we consider the effect of positioning, it quickly becones clear that authors can do a great deal of damage to layout, just as they can do very interesting things. [4] This is usually the case with useful technologies: the sword always has at least two edges, both of them sharp.
设置'[4]'为绝对定位，'top=0 left=0'，效果如下：
when we consider the effect of positioning, it quickly becones clear that authors can do a great deal of damage to layout, just as they can do very interesting things. [4] This is usually the case with useful technologies: the sword always has at least two edges, both of them sharp.
把它的top改为auto：
when we consider the effect of positioning, it quickly becones clear that authors can do a great deal of damage to layout, just as they can do very interesting things. [4] This is usually the case with useful technologies: the sword always has at least two edges, both of them sharp.
可以看到，绝对定位元素的top设置为auto与设置为0的表现是不一样的。设置为auto后，绝对定位元素的 顶端相对于其静态位置对齐。其中静态位置指它未定位前在正常流中的位置。
如果left或right为auto呢，也适用上述规则。这种情况下，定位元素的左（或右）边界与元素未定位时该边界原本的位置对齐。修改上述例子，使top和left都为auto。
when we consider the effect of positioning, it quickly becones clear that authors can do a great deal of damage to layout, just as they can do very interesting things. [4] This is usually the case with useful technologies: the sword always has at least two edges, both of them sharp.
非替换元素的放置和大小
对于定位的非替换元素的宽度和水平放置，这里有一条规则: left + margin-left + border-left-width + padding-left + width + padding-right + border-right-width + margin-right + right = 包含块的width
对于定位的非替换元素的高度和垂直放置，规则与水平规则一样。
原文
如果left,right,width都为auto
如果left,right,width都为auto，就会得到上节的结果：左边界置于其静态位置。
下例，left,width,right为auto，top为0
如果left,right,width都不为auto，左右边距为auto
如果left,right,width都不为auto，左右边距为auto，此时左右边距会设置成相等的值，使元素水平居中。
如果left,right,width,margin-left,margin-right都不为auto
如果left,right,width,margin-left,margin-right都不为auto，此时就会出现过度受限的情况。 此时用户代理会修改right的值，或者忽略right值。
只要有一个水平属性为auto，用户代理就会修改这个auto来适应规则
把上述例子的margin-left设置为auto