为什么默认情况下文本节点呈现在其父级 ::before 下方? [英] Why is a textnode rendered below its parents` ::before by default?

问题描述

在写一个关于 SO 的不同问题的答案时，我做了这个片段:

On writing-up an answer for a different question on SO, I made this snippet:

@import url('https://fonts.googleapis.com/css?family=Shadows+Into+Light');

/* relevant CSS */
div {
  position: relative;
}

div::before {
  content: '';
  position: absolute;
  top: 0; left:0;
}
div>span {
  position:relative;
  z-index:0;
}

/* rest is just styling - should be irrelevant for the question */


div {
  font: normal normal normal 2rem/1 'Shadows Into Light', cursive;
  color: white;
  text-align:center;
  margin: 1rem;
  padding: 1rem;
  min-width: 15rem;
  cursor: pointer;
}
div::before {
  width: 100%;
  height: 100%;
  opacity: 1;
  transition: opacity .3s cubic-bezier(.4,0,.2,1);
  background-color: #bada55;
  border-radius: .4rem;
}
div[orange]:before {
  background-color: #f50;
}
div:hover::before {
  opacity:.65;
}

body {
  margin: 0; padding: 0;
}
center-me-please {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  background: transparent url(http://lorempixel.com/g/1200/800/fashion) no-repeat 50% 50% /cover;
}

<center-me-please>
  
  <div><span>#bada55</span></div>
  <div orange>not so #bada55</div>

我很惊讶地注意到 ::before 元素呈现在文本节点(橙色元素)之上，并且为了防止它发生.我不得不将 textnode 包裹在一个 span 元素中，并给它一个非负的 z-index 和一个非静态的 position (#bada55元素).

I was surprised to notice the ::before element is rendered above textnodes (orange element) and, in order to prevent it from happening. I had to wrap the textnode in a span element and give it a non-negative z-index and a non-static position (#bada55 element).

在检查中，虽然 ::before 元素具有 z-index (auto) 的默认(预期)值，但文本节点似乎根本没有(或者至少 Chrome 无法显示它).

On inspection, while the ::before element has a default (expected) value of z-index (auto), the textnode doesn't seem to have one at all (or at least Chrome's not able to show it).

到目前为止，我喜欢把自己想象成一个 z-index 小忍者，这个想法部分得到了开发 这个玩具帮助朋友和同事更好地理解stacking contexts的原理和z-index.

Up to now I liked to think of myself as a z-index little ninja, idea partly backed-up by developing this toy to help friends and colleagues better understand stacking contexts principle and z-index in general.

正如您可能已经猜到的那样，我正在寻找有关为什么 ::before 默认情况下不呈现在元素中的其他所有内容下方的任何解释(它是第一个，因此在下方，对吗?)和任何关于这是(已知?)错误或有意(设计?)行为的证据.

As you might have guessed, I'm looking for any explanation on why ::before is not rendered below everything else in an element by default (it's first, therefore below, right?) and for any evidence about this being either a (known?) bug or intended (by design?) behavior.

我可能错过或误解的规范会很棒.

A spec I might have missed or misinterpreted would be great.

推荐答案

::before is 默认情况下绘制在文本内容下方——默认情况是当 一切都是非定位的.

::before is painted below text content by default — and the default case is when everything is non-positioned.

但是你的 ::before 是绝对定位的.定位框总是绘制在非定位框的前面.请参阅第 9.9.1 节(重点是我的):

But your ::before is absolutely positioned. Positioned boxes are always painted in front of non-positioned boxes. Refer to section 9.9.1 (emphases mine):

在每个堆叠上下文中，以下图层按从后到前的顺序绘制:

Within each stacking context, the following layers are painted in back-to-front order:

1. 构成堆叠上下文的元素的背景和边框.
2. 具有负堆栈级别(最负优先)的子堆栈上下文.
3. 流入的、非内联级别的、非定位的后代.
4. 未定位的浮动.
5. 内联、内联级、非定位后代，包括内联表和内联块.
6. 堆栈级别为 0 的子堆栈上下文和堆栈级别为 0 的定位后代.
7. 具有正堆栈级别的子堆栈上下文(最不积极的优先).

将文本内容包装在定位的 span 中会导致它按预期绘制在 ::before 内容的前面，从那时起您有两个按源顺序定位的框.

Wrapping your text content in a positioned span causes it to be painted in front of the ::before content as expected since then you have two positioned boxes in source order.

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