margin:auto position: absolute 居中 对应在 w3c规范 中的描述
https://www.w3.org/TR/CSS2/visudet.html
10.6 Calculating heights and margins
For calculating the values of 'top', 'margin-top', 'height', 'margin-bottom', and 'bottom' a distinction must be made between various kinds of boxes:
- inline, non-replaced elements
- inline, replaced elements
- block-level, non-replaced elements in normal flow
- block-level, replaced elements in normal flow
- floating, non-replaced elements
- floating, replaced elements
- absolutely positioned, non-replaced elements
- absolutely positioned, replaced elements
- 'inline-block', non-replaced elements in normal flow
- 'inline-block', replaced elements in normal flow
这里适用于7 8
绝对定位，非替换元素
绝对定位，替换元素
10.6.4 Absolutely positioned, non-replaced elements
For the purposes of this section and the next, the term "static position" (of an element) refers, roughly, to the position an element would have had in the normal flow. More precisely, the static position for 'top' is the distance from the top edge of the containing block to the top margin edge of a hypothetical box that would have been the first box of the element if its specified 'position' value had been 'static' and its specified 'float' had been 'none' and its specified 'clear' had been 'none'. (Note that due to the rules in section 9.7 this might require also assuming a different computed value for 'display'.) The value is negative if the hypothetical box is above the containing block.
But rather than actually calculating the dimensions of that hypothetical box, user agents are free to make a guess at its probable position.
For the purposes of calculating the static position, the containing block of fixed positioned elements is the initial containing block instead of the viewport.
For absolutely positioned elements, the used values of the vertical dimensions must satisfy this constraint:
'top' + 'margin-top' + 'border-top-width' + 'padding-top' + 'height' + 'padding-bottom' + 'border-bottom-width' + 'margin-bottom' + 'bottom' = height of containing block
If all three of 'top', 'height', and 'bottom' are auto, set 'top' to the static position and apply rule number three below.
If none of the three are 'auto': If both 'margin-top' and 'margin-bottom' are 'auto', solve the equation under the extra constraint that the two margins get equal values.
如果这三个（top', 'height', and 'bottom'）都不是“auto”:如果“margin-top”和“margin-bottom”都是“auto”，则在两个边距相等的额外约束下求解方程。
If one of 'margin-top' or 'margin-bottom' is 'auto', solve the equation for that value. If the values are over-constrained, ignore the value for 'bottom' and solve for that value.
Otherwise, pick the one of the following six rules that applies.
- 'top' and 'height' are 'auto' and 'bottom' is not 'auto', then the height is based on the content per 10.6.7, set 'auto' values for 'margin-top' and 'margin-bottom' to 0, and solve for 'top'
- 'top' and 'bottom' are 'auto' and 'height' is not 'auto', then set 'top' to the static position, set 'auto' values for 'margin-top' and 'margin-bottom' to 0, and solve for 'bottom'
- 'height' and 'bottom' are 'auto' and 'top' is not 'auto', then the height is based on the content per 10.6.7, set 'auto' values for 'margin-top' and 'margin-bottom' to 0, and solve for 'bottom'
- 'top' is 'auto', 'height' and 'bottom' are not 'auto', then set 'auto' values for 'margin-top' and 'margin-bottom' to 0, and solve for 'top'
- 'height' is 'auto', 'top' and 'bottom' are not 'auto', then 'auto' values for 'margin-top' and 'margin-bottom' are set to 0 and solve for 'height'
- 'bottom' is 'auto', 'top' and 'height' are not 'auto', then set 'auto' values for 'margin-top' and 'margin-bottom' to 0 and solve for 'bottom'
关于为什么 非position absolute，只能水平居中，不能垂直居中
10.3.3 Block-level, non-replaced elements in normal flow
The following constraints must hold among the used values of the other properties:
'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' = width of containing block
If 'width' is not 'auto' and 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' (plus any of 'margin-left' or 'margin-right' that are not 'auto') is larger than the width of the containing block, then any 'auto' values for 'margin-left' or 'margin-right' are, for the following rules, treated as zero.
If all of the above have a computed value other than 'auto', the values are said to be "over-constrained" and one of the used values will have to be different from its computed value. If the 'direction' property of the containing block has the value 'ltr', the specified value of 'margin-right' is ignored and the value is calculated so as to make the equality true. If the value of 'direction' is 'rtl', this happens to 'margin-left' instead.
If there is exactly one value specified as 'auto', its used value follows from the equality.
If 'width' is set to 'auto', any other 'auto' values become '0' and 'width' follows from the resulting equality.
If both 'margin-left' and 'margin-right' are 'auto', their used values are equal. This horizontally centers水平居中 the element with respect to the edges of the containing block.
这里说明了，可以水平居中
下面
10.6.2 Inline replaced elements, block-level replaced elements in normal flow块级替换元素在标准流, 'inline-block' replaced elements in normal flow and floating replaced elements
If 'margin-top', or 'margin-bottom' are 'auto', their used value is 0.
如果是auto，使用0，所以不能垂直居中
If 'height' and 'width' both have computed values of 'auto' and the element also has an intrinsic height, then that intrinsic height is the used value of 'height'.
Otherwise, if 'height' has a computed value of 'auto', and the element has an intrinsic ratio then the used value of 'height' is:
(used width) / (intrinsic ratio)
Otherwise, if 'height' has a computed value of 'auto', and the element has an intrinsic height, then that intrinsic height is the used value of 'height'.
Otherwise, if 'height' has a computed value of 'auto', but none of the conditions above are met, then the used value of 'height' must be set to the height of the largest rectangle that has a 2:1 ratio, has a height not greater than 150px, and has a width not greater than the device width.
父级元素/父元素 可以为absolute或fixed或relative，注意，并不一定是relative
