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伪类 - CSS（层叠样式表）编辑

CSS 伪类是添加到选择器的关键字，指定要选择的元素的特殊状态。例如，:hover 可被用于在用户将鼠标悬停在按钮上时改变按钮的颜色。

/* 所有用户指针悬停的按钮 */
button:hover {
  color: blue;
}

伪类连同伪元素一起，他们允许你不仅仅是根据文档 DOM 树中的内容对元素应用样式，而且还允许你根据诸如像导航历史这样的外部因素来应用样式（例如 :visited），同样的，可以根据内容的状态（例如在一些表单元素上的 :checked），或者鼠标的位置（例如 :hover 让你知道是否鼠标在一个元素上悬浮）来应用样式。

注意： 与伪类相反，pseudo-elements 可被用于为一个元素的 特定部分 应用样式。

语法

selector:pseudo-class {
  property: value;
}

类似于普通的类，你可以在一个选择器中同时一起写多个伪类。

标准伪类索引

规范

规范状态备注

Fullscreen API	Living Standard	Defined `:fullscreen`.
HTML Living Standard	Living Standard	Defines when particular selectors match HTML elements.
Selectors Level 4	Working Draft	Defined `:any-link`, `:blank`, `:local-link`, `:scope`, `:drop`, `:current`, `:past`, `:future`, `:placeholder-shown`, `:user-invalid`, `:nth-col()`, `:nth-last-col()`, `:is()` and `:where()`. Changed `:empty` to behave like `:-moz-only-whitespace` . No significant change for other pseudo-classes defined in Selectors Level 3 and HTML5 (though semantic meaning not taken over).
HTML5	Recommendation	Copies the relevant section from the canonical (WHATWG) HTML spec.
CSS Basic User Interface Module Level 3	Recommendation	Defined `:default`, `:valid`, `:invalid`, `:in-range`, `:out-of-range`, `:required`, `:optional`, `:read-only` and `:read-write`, but without the associated semantic meaning.
Selectors Level 3	Recommendation	Defined `:target`, `:root`, `:nth-child()`, `:nth-last-of-child()`, `:nth-of-type()`, `:nth-last-of-type()`, `:last-child`, `:first-of-type`, `:last-of-type`, `:only-child`, `:only-of-type`, `:empty` and `:not()`. Defined the syntax of `:enabled`, `:disabled`, `:checked`, and `:indeterminate`, but without the associated semantic meaning. No significant change for pseudo-classes defined in CSS Level 2 (Revision 1).
CSS Level 2 (Revision 1)	Recommendation	Defined `:lang()`, `:first-child`, `:hover`, and `:focus`. No significant change for pseudo-classes defined in CSS Level 1.
CSS Level 1	Recommendation	Defined `:link`, `:visited` and `:active`, but without the associated semantic meaning.