css 文档流中块级非替换元素水平区域的计算规则(1)_html/css_WEB-ITnose
最近在读《Basic Visual Formatting in CSS》，结合以前看的《css权威指南》和css标准。今天就做个笔记。
以前在遇到一些宽度不明确指明的一些布局的时候，虽然凭感觉能猜出个大概，但是总是有点不是很靠谱。直到最近看到这一本书，觉得总觉得挺好的。
首先要知道，元素除了通过display来指定block、inline、inline-block这一些属性之外，还有一种能在的特性--替换还是非替换。
替换元素：像img、video、canvas等稳定种指定的内容只是占位，真正的内容实在后面替换上去的。像img内容是图片加载完成之后替换上去的。
非替换元素：像div、p、span这一些内容就写在文档中。
而这里要说到的主要是块级非替换元素既display：block的非替换元素，如p、div；还有就是display：list-item，如li。li虽然看display像是和div、p这一些是两类，但是其实li除了多了前面的标志之外，别的行为基本都是一样的。
下面进入正题，文档流中块级非替换元素的水平格式化和包含块（containing block）还有七个属性有关（border-left、margin-left、padding-left、width、padding-right、margin-right、border-right）。文档流中块级元素的containing block一般指的是父级块级元素的内容区域（content area）。也就是在box-sizing：content-box时，width指定的区域。下面有一条公式：
'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' = width of containing block
上面的公式在所有情况下都要成立。但是很快问题就来了，要是我把7个属性的值都设了值，但是加起来不等于containing block的width怎么办；又或者我把值都设置为auto，那又怎么样去计算呢？其实在这七个值中，只有三个值margin-left、margin-right、width能够设置为auto。我们首先看看把这三个属性设置为auto的情况，由于这三个属性可以设置为auto或者是非auto，那么就是说一共有23，8种情况。下面来分别说下：
1、一个值设置为auto，也就是margin-left、margin-right或者width设置为auto。这三种情况最简单了，直接套上面的公式一算就知道了。
2、两个值设置为auto。
2.1、margin-left和margin-right设置为auto，width指定一个非auto的值。这种情况就是我们平时的水平居中，margin-left和margin-right获得相同的值。
2.2、margin-left和width设置为auto，margin-right指定一个非auto的值。margin-left重设为0，然后就变成了1情况了。
2.3、margin-right和width设置为auto，margin-left指定一个非auto的值。margin-right重设为0，然后又变成了1情况了。
3、三个值设置为auto。margin-left和margin-right都重设为0，然后根据1来计算。
4、假如没有一个设置为auto，在css中叫"over-constrained"（过度约束）。那么在文档流方向为左到右的时候margin-right的值会被忽略，然后还是1情况。这种情况最容易理解错，容易吧"over-constrained"理解成指定了所有的值后使得上面公式左边小于右边。正确的理解应该是"指定了三个值，都没有变通的余地了"。所以只要给margin-left、width和margin-right都指定了auto之外的值（就算是0），也是"over-constrained"了。
上面的内容似乎已经覆盖了所有的情况了，其实不然，还有一种特殊情况就是margin-left，margin-right设置为auto，width有指定值，但是border-left+padding-left+width+padding-right+border-right> width of containing block。按照上面的说法，margin-left和margin-right获得相同的值，但事实是，margin-left和margin-right重设为0，然后过度约束，然后忽略掉一个。
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