## 伪类
>The pseudo-class concept is introduced to permit selection based on information that lies outside of the document tree or that cannot be expressed using the other simple selectors.
## 伪元素
>Pseudo-elements create abstractions about the document tree beyond those specified by the document language. For instance, document languages do not offer mechanisms to access the first letter or first line of an element's content. Pseudo-elements allow authors to refer to this otherwise inaccessible information. Pseudo-elements may also provide authors a way to refer to content that does not exist in the source document (e.g., the ::before and ::after pseudo-elements give access to generated content).
[浅谈CSS中的伪元素和伪类](https://www.w3cschool.cn/css_series/css_series-pwkt24q2.html)
注：css2之前伪元素和伪类都用一个冒号表示，但是css3之后为了区分两者，伪类采用一个冒号来表示，伪元素采用两个冒号来表示