:before 和 ::before 有什么区别？
我刚刚看到一个包含 ::before 标签的 CSS 代码。我查看了 MDN 以了解 ::before是但我真的不明白。
有人可以解释它是如何工作的吗？
它会在我们通过 CSS 选择元素之前创建 DOM 元素吗？
I just saw a CSS code that included ::before tag. I looked at MDN to see what the ::before is but I really didn't understand it.
Can someone explain how it works?
Does it make a DOM element before what we select by CSS?
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根据这些文档，它们是等效的：
唯一的区别是 CSS3 中使用双冒号，而单冒号是旧版本。
推理：
According to those docs, they are equivalent:
The only difference is that the double colon is used in CSS3, whereas the single colon is the legacy version.
Reasoning:
这将伪元素与伪类区分开来。
伪类和伪元素之间的区别在 http://www 中描述.d.umn.edu/~lcarlson/csswork/selectors/pseudo_dif.html
This distinguishes pseudo elements from pseudo classes.
The difference between pseudo classes and pseudo elements is described at http://www.d.umn.edu/~lcarlson/csswork/selectors/pseudo_dif.html
它们本质上意味着同一件事。 CSS3 中引入了
::来帮助区分伪元素（如 :before 和 :after）和伪类（如 :link 和 :hover）。They essentially mean the same thing. The
::was introduced in CSS3 to help descriminate between pseudo elements (like :before and :after) and pseudo classes (like :link and :hover).我查看了 MDN 和 w3.org，我能想到的最好的办法是
::用于结构< /strong> 改变，以及:用于样式。出于兼容性原因，它们目前可以互换。
它似乎将
:link（例如）（它的样式为）与:before（这是一个 结构变化）。:用于样式，::用于结构。I checked out MDN and w3.org, and the best I could come up with is that
::is used for structural changes, and:is used for styling.They are currently interchangeable for compatibility reasons.
It appears to separate
:link(for instance), which styles a<a>, from:before(which is a structural change).:is for styling,::is for structure.一种是 CSS2 (:before) 方式，另一种是 CSS3 (::before) 方式。目前，它们在支持 CSS2 和 CSS2 的浏览器中是可以互换的。 CSS3。
这是一个很好的解释： http://www.impressivewebs.com/before-after-css3/< /a>
One is the CSS2 (:before) way and the other is CSS3 (::before). Currently they are interchangeable in browsers that support CSS2 & CSS3.
Here's a good explanation: http://www.impressivewebs.com/before-after-css3/