阅读 W3C 2018年9月11日发布的 Selectors Level 3 提议的一些翻译和记录，文档中说，这份提议还只是草稿，不属于规范的一部分。
概要
选择器（Selectors ）是一种用来匹配树节点元素的模式，因此一种可以用来选择XML文档节点的技术。选择器已经针对 HTML 和 XML 进行了优化，旨在用于性能苛刻的代码。
CSS (Cascading Style Sheets) 是一种用于描述在显示器、纸张和语音等中渲染和呈现 HTML 和 XML 文 档的语言。CSS 用选择器将样式属性绑定到文档的元素。
在 CSS1 和 CSS2 中已经有了选择器，下面介绍 CSS3 以及其他语言可能需要用到的选择器。
选择器定义了下面的方法：1
expression * element -> boolean
就是说，给定一个元素和一个选择器，规范定义改元素是否与选择器匹配。
通过对一个子树中的所有元素进行表达式评估，这些表达式还可以被用于选择一些元素集合，或者从元素集合中选择一个元素。
介绍
一级选择器（Selector Level 1）和二级选择器（Selector Level 2）已经分别被定义为 CSS1 和 CSS2.1规范中定义的选择器功能的子集。
相对 CSS2 的变化
- 一些基本的定义（selector，group of selector,simple selector等）发生了变化；尤其是，在 CSS2 中的简单选择器现在称为简单选择器序列（sequence of simple selectors）,术语“简单选择器” 现在被用于该序列的组件。
- 可选的命名空间组件现在可以被用于元素类型选择器，通用选择器和属性选择器。
- 引入了新的组合器——后续兄弟组合（Subsequent-sibling combinator）
- 新的简单选择器，包括子串匹配属性选择器（substring matching attribute）和新的伪类（pseudo-classes）
- 新的伪元素，以及伪元素的 “::” 约定的引入。
- 语法被重写
- 选择器现在是 CSS3 的模块，有独立的声明，其他选择器可以独立于 CSS 参考本选择器的文档。
选择器（Selector）
下面是选择器语法的总结
| Pattern | Represents | Description | Level | |
|---|---|---|---|---|
| * | any element | Universal selector | 2 | |
| E | an element of type E | Type selector | 1 | |
| E[foo] | an E element with a “foo” attribute | Attribute selectors | 2 | |
| E[foo=”bar”] | an E element whose “foo” attribute value is exactly equal to “bar” | Attribute selectors | 2 | |
| E[foo~=”bar”] | an E element whose “foo” attribute value is a list of whitespace-separated values, one of which is exactly equal to “bar” | Attribute selectors | 2 | |
| E[foo^=”bar”] | an E element whose “foo” attribute value begins exactly with the string “bar” | Attribute selectors | 3 | |
| E[foo$=”bar”] | an E element whose “foo” attribute value ends exactly with the string “bar” | Attribute selectors | 3 | |
| E[foo*=”bar”] | an E element whose “foo” attribute value contains the substring “bar” | Attribute selectors | 3 | |
| E[foo\ | =”en”] | an E element whose “foo” attribute has a hyphen-separated list of values beginning (from the left) with “en” | Attribute selectors | 2 |
| E:root | an E element, root of the document | Structural pseudo-classes | 3 | |
| E:nth-child(n) | an E element, the n-th child of its parent | Structural pseudo-classes | 3 | |
| E:nth-last-child(n) | an E element, the n-th child of its parent, counting from the last one | Structural pseudo-classes | 3 | |
| E:nth-of-type(n) | an E element, the n-th sibling of its type | Structural pseudo-classes | 3 | |
| E:nth-last-of-type(n) | an E element, the n-th sibling of its type, counting from the last one | Structural pseudo-classes | 3 | |
| E:first-child | an E element, first child of its parent | Structural pseudo-classes | 2 | |
| E:last-child | an E element, last child of its parent | Structural pseudo-classes | 3 | |
| E:first-of-type | an E element, first sibling of its type | Structural pseudo-classes | 3 | |
| E:last-of-type | an E element, last sibling of its type | Structural pseudo-classes | 3 | |
| E:only-child | an E element, only child of its parent | Structural pseudo-classes | 3 | |
| E:only-of-type | an E element, only sibling of its type | Structural pseudo-classes | 3 | |
| E:empty | an E element that has no children (including text nodes) | Structural pseudo-classes | 3 | |
| E:link E:visited | an E element being the source anchor of a hyperlink of which the target is not yet visited (:link) or already visited (:visited) | The link pseudo-classes | 1 | |
| E:active E:hover E:focus | an E element during certain user actions | The user action pseudo-classes | 1 and 2 | |
| E:target | an E element being the target of the referring URI | The target pseudo-class | 3 | |
| E:lang(fr) | an element of type E in language “fr” (the document language specifies how language is determined) | The :lang() pseudo-class | 2 | |
| E:enabled E:disabled | a user interface element E which is enabled or disabled | The UI element states pseudo-classes | 3 | |
| E:checked | a user interface element E which is checked (for instance a radio-button or checkbox) | The UI element states pseudo-classes | 3 | |
| E::first-line | the first formatted line of an E element | The ::first-line pseudo-element | 1 | |
| E::first-letter | the first formatted letter of an E element | The ::first-letter pseudo-element | 1 | |
| E::before | generated content before an E element | The ::before pseudo-element | 2 | |
| E::after | generated content after an E element | The ::after pseudo-element | 2 | |
| E.warning | an E element whose class is “warning” (the document language specifies how class is determined). | Class selectors | 1 | |
| E#myid | an E element with ID equal to “myid”. | ID selectors | 1 | |
| E:not(s) | an E element that does not match simple selector s | Negation pseudo-class | 3 | |
| E F | an F element descendant of an E element | Descendant combinator | 1 | |
| E > F | an F element child of an E element | Child combinator | 2 | |
| E + F | an F element immediately preceded by an E element | Next-sibling combinator | 2 | |
| E ~ F | an F element preceded by an E element | Subsequent-sibling combinator | 3 |
区分大小写
所有的选择器语法都是在 ASCII 范围内且大小不写敏感的（例如 [a-z] 和 [A-Z]是等同的），除了不受选择器控制的部分。在选择器中的文档语言元素名称，属性名称和属性值的是否区分大小写要取决于文档语言本身。例如，在 HTML 中，元素名是大小写敏感的，但是 XML 中区分大小写。