CSS中可以单独使用伪元素吗？

Question

根据 W3C，选择器的定义不包括伪元素： https://www.w3.org/TR/css3-selectors/#selector-syntax

上面link说：

A selector is a chain of one or more sequences of simple selectors separated by combinators.

它还说：

A simple selector is either a type selector, universal selector, attribute selector, class selector, ID selector, or pseudo-class.

关于如何使用伪元素，它说：

One pseudo-element may be appended to the last sequence of simple selectors in a selector.

和

Only one pseudo-element may appear per selector, and if present it must appear after the sequence of simple selectors that represents the subjects of the selector.

那么这是否意味着伪元素只能作为 "valid" 选择器的后缀而不应该单独使用？

Answer 1

does that mean that a pseudo-element can only be a suffix to a "valid" selector and should not be used alone?

你的结论是不正确的，因为universal selector*可以省略

If a universal selector represented by * [...] is immediately followed by a pseudo-element, then the * may be omitted and the universal selector's presence implied.

所以你可以单独使用伪元素，例如::before，因为在引擎盖下它将被视为 *::before。

::before {
  content: 'Hello!';
}