CSS Selector（选择器）
W3C标准手册中的选择器如下表（共52个）：
| Selector | Example | Example description | CSS |
|---|---|---|---|
| .class | .intro | Selects all elements with class="intro" | 1 |
| #id | #firstname | Selects the element with id="firstname" | 1 |
| * | * | Selects all elements | 2 |
| element | p | Selects all <p> elements | 1 |
| element,element | div,p | Selects all <div> elements and all <p> elements | 1 |
| element element | div p | Selects all <p> elements inside <div> elements | 1 |
| element>element | div>p | Selects all <p> elements where the parent is a <div> element | 2 |
| element+element | div+p | Selects all <p> elements that are placed immediately after <div> elements | 2 |
| element1~element2 | p~ul | Selects every <ul> element that are preceded by a <p> element | 3 |
| [attribute] | [target] | Selects all elements with a target attribute | 2 |
| [attribute=value] | [target=_blank] | Selects all elements with target="_blank" | 2 |
| [attribute~=value] | [title~=flower] | Selects all elements with a title attribute containing the word "flower" | 2 |
| [attribute|=value] | [lang|=en] | Selects all elements with a lang attribute value starting with "en" | 2 |
| [attribute^=value] | a[src^="https"] | Selects every <a> element whose src attribute value begins with "https" | 3 |
| [attribute$=value] | a[src$=".pdf"] | Selects every <a> element whose src attribute value ends with ".pdf" | 3 |
| [attribute*=value] | a[src*="w3schools"] | Selects every <a> element whose src attribute value contains the substring "w3schools" | 3 |
| :active | a:active | Selects the active link | 1 |
| ::after | p::after | Insert content after every <p> element | 2 |
| ::before | p::before | Insert content before the content of every <p> element | 2 |
| :checked | input:checked | Selects every checked <input> element | 3 |
| :disabled | input:disabled | Selects every disabled <input> element | 3 |
| :empty | p:empty | Selects every <p> element that has no children (including text nodes) | 3 |
| :enabled | input:enabled | Selects every enabled <input> element | 3 |
| :first-child | p:first-child | Selects every <p> element that is the first child of its parent | 2 |
| ::first-letter | p::first-letter | Selects the first letter of every <p> element | 1 |
| ::first-line | p::first-line | Selects the first line of every <p> element | 1 |
| :first-of-type | p:first-of-type | Selects every <p> element that is the first <p> element of its parent | 3 |
| :focus | input:focus | Selects the input element which has focus | 2 |
| :hover | a:hover | Selects links on mouse over | 1 |
| :in-range | input:in-range | Selects input elements with a value within a specified range | 3 |
| :invalid | input:invalid | Selects all input elemets with an invalid value | 3 |
| :lang(language) | p:lang(it) | Selects every <p> element with a lang attribute equal to "it" (Italian) | 2 |
| :last-child | p:last-child | Selects every <p> element that is the last child of its parent | 3 |
| :last-of-type | p:last-of-type | Selects every <p> element that is the last <p> element of its parent | 3 |
| :link | a:link | Selects all unvisited links | 1 |
| :not(selector) | :not(p) | Selects every element that is not a <p> element | 3 |
| :nth-child(n) | p:nth-child(2) | Selects every <p> element that is the second child of its parent | 3 |
| :nth-last-child(n) | p:nth-last-child(2) | Selects every <p> element that is the second child of its parent, counting from the last child | 3 |
| :nth-last-of-type(n) | p:nth-last-of-type(2) | Selects every <p> element that is the second <p> element of its parent, counting from the last child | 3 |
| :nth-of-type(n) | p:nth-of-type(2) | Selects every <p> element that is the second <p> element of its parent | 3 |
| :only-of-type | p:only-of-type | Selects every <p> element that is the only <p> element of its parent | 3 |
| :only-child | p:only-child | Selects every <p> element that is the only child of its parent | 3 |
| :optional | input:optional | Selects input elements with no "required" attribute | 3 |
| :out-of-range | input:out-of-range | Selects input elements with a value outside a specified range | 3 |
| :read-only | input:read-only | Selects input elements with the "readonly" attribute specified | 3 |
| :read-write | input:read-write | Selects input elements with the "readonly" attribute NOT specified | 3 |
| :required | input:required | Selects input elements with the "required" attribute specified | 3 |
| :root | :root | Selects the document's root element | 3 |
| ::selection | ::selection | Selects the portion of an element that is selected by a user | |
| :target | #news:target | Selects the current active #news element (clicked on a URL containing that anchor name) | 3 |
| :valid | input:valid | Selects all input elements with a valid value | 3 |
| :visited | a:visited | Selects all visited links | 1 |
=============================基本选择器=========================
一、类选择器（.className）
在 CSS 中，类选择器以一个点号显示：
.center {text-align: center}
所有拥有 center 类的 HTML 元素均居中。
<h1 class="center"> This heading will be center-aligned </h1> <p class="center"> This paragraph will also be center-aligned. </p>
h1 和 p 元素都有 center 类，两者将遵守 ".center" 选择器中的规则
p.important {color:red;}
匹配 class 属性包含 important 的所有 p 元素，但是其他任何类型的元素都不匹配，不论是否有此 class 属性
<p class="important warning"> This paragraph is a very important warning. </p> .important {font-weight:bold;} .warning {font-style:italic;} .important.warning {background:silver;}
这两个词的顺序无关紧要，写成 warning important 也可以
二、ID选择器（#id）
<p class="content" id="post"></p> <div class="content"></div> <div id="post"></div> <!--id="post"重复出现不允许-->
在设置id选择器css时，必须在选择器名称前加”#”，形如：
#post{ color:#000; }
<html> <head> <title>css id选择器</title> <style type="text/css"> #blue{ color:blue; } #large{ font-weight:bold; } </style> </head> <body > <p id="large">我被加粗了。</p> <p id="blue">我被设置成蓝色</p> <p id="blue">我被设置成蓝色</p> <p id="blue large">我没有被设置css样式</p> </body> </html>
提示：第3个p标签重复使用了id=”blue”，css设置仍然生效，不过这种做法是不允许的。因为在一个网页中，往往除了html代码还包含其它脚本语言。例如使用javascript脚本的网页，如果相同的id属性重复使用则会出错。所以我们应该要记住：id属性不能重复定义。第4个p标签的css样式没有生效是因为不允许对一个标签定义两个id属性
三、通配符选择器（*）
该选择器可以选中所有的HTML元素，但其优先级最低：Id选择器> class选择器> html选择器> 通配符选则器
*{ margin-top:0px; margin-left:0px; margin:10px 30px 20px 40px; margin:0px; padding:0px; }
通常用于页面初始化，会降低浏览器渲染页面的速度
四、元素选择器（E）
最常见的 CSS 选择器是元素选择器。换句话说，文档的元素就是最基本的选择器。
如果设置 HTML 的样式，选择器通常将是某个 HTML 元素，比如 p、h1、em、a，甚至可以是 html 本身：
html {color:black;} h1 {color:blue;} h2 {color:silver;}
可以将某个样式从一个元素切换到另一个元素。
假设您决定将上面的段落文本（而不是 h1 元素）设置为灰色。只需要把 h1 选择器该为 p：
html {color:black;} p {color:gray;} h2 {color:silver;}
=============================关系选择器=========================
