问题描述
这是我正在看的整行代码，这里是它的上下文：
This is the full line of code I'm looking at, and here is its context: http://acidmartin.wordpress.com/2011/02/24/custom-crossbrowser-styling-for-checkboxes-and-radio-buttons
input[type="radio"] + span::before { content: ""; display: inline-block; width: 20px; height: 20px; background: url("sprite.png") no-repeat -20px 0; vertical-align: middle; }
我有一个很好的理解这是如何工作，但我不明白为什么有两个冒号，而不是span和之前的冒号。
I have a decent understanding of how this works, but I don't understand why there are two colons, rather than one between span and before.
前面的选择器，从我读过的应该使用一个冒号。
The before selector, from what I've read should use one colon.
在w3c上，我找不到有两个冒号的选择器，也不知道为什么span会有冒号，以及之前的冒号。
On w3c, I can't find any selectors that have two colons, nor can I figure out why span would have a colon following it, in addition to the colon preceding "before".
推荐答案
这是一个伪元素，由规范定义：
It's a pseudo-element, as defined by the CSS Selectors Level 3 spec:
它实际上与由第2级规范定义的单冒号语法。第3级规范引入了一个额外的冒号来区分伪元素和伪类（使用单个冒号）。
It is effectively the same as the single-colon syntax defined by the level 2 spec. The level 3 spec introduces an extra colon to differentiate between pseudo-elements and pseudo-classes (which use a single colon).
这两种语法都适用于较新的浏览器，但是旧版浏览器不会识别较新的 :: 样式。
Both syntaxes will work in newer browsers, but older browsers will not recognise the newer :: style.
有关更详细的信息，您可以查看第3级规范中的，其中规定：
For even more detail, you can look at the grammar from the level 3 spec, which states:
这篇关于为什么这里有两个冒号？ span :: before的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！