为什么transform对行内元素不生效

注：赶时间的同学可直接下拉到底，看结论。

我使用transform对一个元素进行位移，代码如下：

<div class="box">
  <span>今天你吃了么？</span>
</div>

// css
span {
  transform: translateX(20px)
}

然而span标签并没有向右移动20px，这使我感到困惑。

但当我给span增加display: inline-block时，transform又表现正常了——span向右位移了20px。

transform不支持行内元素么？

此时必须google一下啊，果不其然，早有前辈提出了相同的问题：css3-transform-not-working

其中一个回答引用了w3c关于transform的规范：

CSS Transforms Module Level 1 - Transformable Element
A transformable element is an element in one of these categories:

• an element whose layout is governed by the CSS box model which is either a block-level or atomic inline-level element, or whose display property computes to table-row, table-row-group, table-header-group, table-footer-group, table-cell, or table-caption [CSS2]
• an element in the SVG namespace and not governed by the CSS box model which has the attributes transform, patternTransform or gradientTransform [SVG11].

上面说到能够transform的元素有哪些。其中提到atomic inline-level element(原子行内级元素，嗯，翻译就是如此蹩脚)，难不成span、a等行内元素不属于原子行内级元素？

原子行内级元素是什么

An inline box is one that is both inline-level and whose contents participate in its containing inline formatting context. A non-replaced element with a 'display' value of 'inline' generates an inline box. Inline-level boxes that are not inline boxes (such as replaced inline-level elements, inline-block elements, and inline-table elements) are called atomic inline-level boxes because they participate in their inline formatting context as a single opaque box. (https://www.w3.org/TR/CSS2/visuren.html#x13)

上面这段话讲了两个盒子：inline box(行内盒子)和inline-level box(行内级盒子)：

• 行内盒子由一个display:inline的非替换元素生成。
• 行内级盒子又叫做原子行内级盒子，它不是 行内盒子。因为它是透明的。

结论

因为，

display: block: 让一个元素生成一个block box
display: inline-block: 生成一个inline-level block container，元素本身被格式化为atomic inline-level box，它的内部被格式化为block box
display: inline: 让一个元素生成一个或多个inline boxes

而，可被transform的元素有：block-level elementoratomic inline-level elementetc，但不包括inline element.

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