谈一谈CSS的选择器和工作原理

深碍√TFBOYSˉ_ 2022-06-12 10:20 287阅读 0赞

既然我们是谈CSS，那我们先来说一下什么是CSS？

层叠样式表(英文全称：Cascading Style Sheets)是一种用来表现HTML（标准通用标记语言的一个应用）或XML（标准通用标记语言的一个子集）等文件样式的计算机语言。CSS不仅可以静态地修饰网页，还可以配合各种脚本语言动态地对网页各元素进行格式化。CSS 能够对网页中元素位置的排版进行像素级精确控制，支持几乎所有的字体字号样式，拥有对网页对象和模型样式编辑的能力。

之所以叫“层叠”，就是因为在 CSS 中，一个文件的样式可以从其他的样式表中继承下来。读者在有些地方可以使用他自己更喜欢的样式，在其他地方则继承，或“层叠”作者的样式,这种层叠的方式使作者和读者都可以灵活地加入自己的设计，混合各人的爱好。哈哈，是不是觉得设计得很美妙……O(∩\_∩)O哈哈~

做个web前端开发的人都知道，css提供了大量的选择器。在 CSS 中，选择器是一种模式，用于选择需要添加样式的元素。

"CSS" 列指示该属性是在哪个 CSS 版本中定义的。

<table style="margin:15px 0px 0px; padding:0px; border:1px solid rgb(170,170,170); border-collapse:collapse; width:809px; color:rgb(0,0,0); font-family:PingFangSC-Regular,Verdana,Arial,微软雅黑,宋体; font-size:14px; background-color:rgb(253,252,248)"> 
 <tbody style="margin:0px; padding:0px; border:0px"> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <th style="margin:0px; padding:5px 15px 5px 6px; border:1px solid rgb(63,63,63); vertical-align:baseline; background-color:rgb(63,63,63); text-align:left; color:rgb(255,255,255)"> 选择器</th> 
 <th style="margin:0px; padding:5px 15px 5px 6px; border:1px solid rgb(63,63,63); vertical-align:baseline; background-color:rgb(63,63,63); text-align:left; color:rgb(255,255,255)"> 例子</th> 
 <th style="margin:0px; padding:5px 15px 5px 6px; border:1px solid rgb(63,63,63); vertical-align:baseline; background-color:rgb(63,63,63); text-align:left; color:rgb(255,255,255)"> 例子描述</th> 
 <th style="margin:0px; padding:5px 15px 5px 6px; border:1px solid rgb(63,63,63); vertical-align:baseline; background-color:rgb(63,63,63); text-align:left; color:rgb(255,255,255); width:31px"> CSS</th> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_class.asp" title="CSS .class 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">.class</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> .intro</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择 class="intro" 的所有元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_id.asp" title="CSS #id 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">#id</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> #firstname</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择 id="firstname" 的所有元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_all.asp" title="CSS * 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">*</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> *</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择所有元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_element.asp" title="CSS element 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">element</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择所有 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_element_comma.asp" title="CSS element,element 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">element,element</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> div,p</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择所有 &lt;div&gt; 元素和所有 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_element_element.asp" title="CSS element element 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">element&nbsp;element</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> div p</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择 &lt;div&gt; 元素内部的所有 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_element_gt.asp" title="CSS element>element 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">element&gt;element</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> div&gt;p</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择父元素为 &lt;div&gt; 元素的所有 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_element_plus.asp" title="CSS element+element 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">element+element</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> div+p</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择紧接在 &lt;div&gt; 元素之后的所有 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_attribute.asp" title="CSS [attribute] 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">[attribute]</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> [target]</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择带有 target 属性所有元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_attribute_value.asp" title="CSS [attribute=value] 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">[attribute=value]</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> [target=_blank]</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择 target="_blank" 的所有元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_attribute_value_contain.asp" title="CSS [attribute~=value] 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">[attribute~=value]</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> [title~=flower]</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择 title 属性包含单词 "flower" 的所有元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_attribute_value_start.asp" title="CSS [attribute|=value] 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">[attribute|=value]</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> [lang|=en]</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择 lang 属性值以 "en" 开头的所有元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_link.asp" title="CSS :link 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:link</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> a:link</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择所有未被访问的链接。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_visited.asp" title="CSS :visited 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:visited</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> a:visited</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择所有已被访问的链接。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_active.asp" title="CSS :active 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:active</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> a:active</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择活动链接。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_hover.asp" title="CSS :hover 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:hover</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> a:hover</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择鼠标指针位于其上的链接。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_focus.asp" title="CSS :focus 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:focus</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> input:focus</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择获得焦点的 input 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_first-letter.asp" title="CSS :first-letter 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:first-letter</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:first-letter</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择每个 &lt;p&gt; 元素的首字母。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_first-line.asp" title="CSS :first-line 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:first-line</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:first-line</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择每个 &lt;p&gt; 元素的首行。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 1</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_first-child.asp" title="CSS :first-child 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:first-child</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:first-child</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择属于父元素的第一个子元素的每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_before.asp" title="CSS :before 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:before</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:before</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 在每个 &lt;p&gt; 元素的内容之前插入内容。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_after.asp" title="CSS :after 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:after</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:after</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 在每个 &lt;p&gt; 元素的内容之后插入内容。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_lang.asp" title="CSS :lang(language) 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:lang(language)</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:lang(it)</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择带有以 "it" 开头的 lang 属性值的每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 2</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_gen_sibling.asp" title="CSS element1~element2 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">element1~element2</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p~ul</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择前面有 &lt;p&gt; 元素的每个 &lt;ul&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_attr_begin.asp" title="CSS [attribute^=value] 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">[attribute^=value]</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> a[src^="https"]</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择其 src 属性值以 "https" 开头的每个 &lt;a&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_attr_end.asp" title="CSS [attribute$=value] 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">[attribute$=value]</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> a[src$=".pdf"]</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择其 src 属性以 ".pdf" 结尾的所有 &lt;a&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_attr_contain.asp" title="CSS [attribute*=value] 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">[attribute*=value]</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> a[src*="abc"]</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择其 src 属性中包含 "abc" 子串的每个 &lt;a&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_first-of-type.asp" title="CSS :first-of-type 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:first-of-type</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:first-of-type</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择属于其父元素的首个 &lt;p&gt; 元素的每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_last-of-type.asp" title="CSS :last-of-type 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:last-of-type</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:last-of-type</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择属于其父元素的最后 &lt;p&gt; 元素的每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_only-of-type.asp" title="CSS :only-of-type 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:only-of-type</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:only-of-type</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择属于其父元素唯一的 &lt;p&gt; 元素的每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_only-child.asp" title="CSS :only-child 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:only-child</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:only-child</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择属于其父元素的唯一子元素的每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_nth-child.asp" title="CSS :nth-child(n) 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:nth-child(n)</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:nth-child(2)</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择属于其父元素的第二个子元素的每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_nth-last-child.asp" title="CSS :nth-last-child(n) 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:nth-last-child(n)</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:nth-last-child(2)</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 同上，从最后一个子元素开始计数。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_nth-of-type.asp" title="CSS :nth-of-type(n) 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:nth-of-type(n)</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:nth-of-type(2)</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择属于其父元素第二个 &lt;p&gt; 元素的每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_nth-last-of-type.asp" title="CSS :nth-last-of-type(n) 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:nth-last-of-type(n)</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:nth-last-of-type(2)</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 同上，但是从最后一个子元素开始计数。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_last-child.asp" title="CSS :last-child 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:last-child</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:last-child</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择属于其父元素最后一个子元素每个 &lt;p&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_root.asp" title="CSS :root 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:root</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> :root</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择文档的根元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_empty.asp" title="CSS :empty 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:empty</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> p:empty</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择没有子元素的每个 &lt;p&gt; 元素（包括文本节点）。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_target.asp" title="CSS :target 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:target</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> #news:target</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择当前活动的 #news 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_enabled.asp" title="CSS :enabled 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:enabled</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> input:enabled</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择每个启用的 &lt;input&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_disabled.asp" title="CSS :disabled 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:disabled</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> input:disabled</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择每个禁用的 &lt;input&gt; 元素</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_checked.asp" title="CSS :checked 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:checked</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> input:checked</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择每个被选中的 &lt;input&gt; 元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(255,255,255)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_not.asp" title="CSS :not(selector) 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">:not(selector)</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> :not(p)</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择非 &lt;p&gt; 元素的每个元素。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; background-color:rgb(245,245,245)"> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> <a href="http://www.w3school.com.cn/cssref/selector_selection.asp" title="CSS ::selection 选择器" style="margin:0px; padding:0px; border-width:0px 0px 1px; border-top-style:initial; border-right-style:initial; border-bottom-style:solid; border-left-style:initial; border-top-color:initial; border-right-color:initial; border-bottom-color:rgb(144,11,9); border-left-color:initial; color:rgb(144,11,9); background:transparent" rel="nofollow">::selection</a></td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> ::selection</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 选择被用户选取的元素部分。</td> 
 <td style="margin:0px; padding:6px 15px 6px 6px; border:1px solid rgb(170,170,170); vertical-align:text-top"> 3</td> 
 </tr> 
 </tbody> 
</table>

但是到现在为止，有时候已有的选择器还有不够完美的地方，比如还不存在下面的几款

child < parent 即根据已知的子节点找到他的直接父节点，跟 > 选择直接孩子结点对应

child $ ancestors 即选择孩子结点的所以祖先结点们

A - B 即选择A节点前面的兄弟节点B，类似于jquery里的A.prev()，与现有的 + 选择后面的兄弟结点相对应

但是查阅W3C相关资料得知，CSS一直几乎没有这样的选择器(现在是没有，希望以后有啊……哈哈^\_^)，是因为这样的选择器会导致回溯。

那问题来了，什么是回溯呢？

那还得从浏览器是如何加载css讲起。浏览器最初设计的时候就考虑了渐进显示，也就是整个文档加载了多少就显示多少内容，而不用等整个下载完。浏览器下载的顺序是从上到下，渲染的顺序也是从上到下，下载和渲染是同时进行的。在渲染到页面的某一部分时，其上面的所有部分都已经下载完成（并不是说所有相关联的元素都已经下载完）。如果遇到语义解释性的标签嵌入文件（JS脚本，CSS样式），那么此时IE的下载过程会启用单独连接进行下载。并且在下载后进行解析，解析过程中，停止页面所有往下元素的下载。 样式表在下载完成后，将和以前下载的所有样式表一起进行解析，解析完成后，将对此前所有元素（含以前已经渲染的）重新进行渲染。渐进显示在CSS上的原理就是一个节点所适用的样式只取决于它和它之前的节点（父节点、它之前的兄弟节点）的性质。而我们刚才提到的几款选择器则恰好相反，跟这个是背道而行的。言外之意，就是说当浏览器解析到一个新节点时，可能改变之前节点所适用的样式——因而要求在解析一个新节点后，得回头重新计算之前节点所匹配的样式，此即所谓“回溯”（就像我们现实中的水再往回流了，O(∩\_∩)O哈哈~）。在最坏的情况下所导致大量的重新计算和reflow，可以相当于重新加载整个网页，在某种程度上来说，这是挺耗资源的。

现在我们来看下CSS是怎么工作的？

CSS 语言是一种标记语言，它不需要编译，可以直接由浏览器解释执行。浏览器CSS匹配不是从左到右进行查找，而是从右到左进行查找。比如 div\#box p span.red\{color:red;\}，浏览器的查找顺序如下：先查找 html 中所有 class=’red’ 的 span 元素，找到后，再查找其父辈元素中是否有p元素，再判断p的父元素中是否有 id 为 box 的 div 元素，如果都存在，则 CSS 匹配上。

浏览器从右到左进行查找的好处是为了尽早过滤掉一些无关的样式规则和元素。Firefox称这种查找方式为 keyselector(关键字查询)，所谓的关键字就是样式规则中最后(最右边)的规则，上面的 key 就是 span.red。

最后谈一下常见的几种CSS优化：

1、不要在id选择器前使用标签名

一般写法：div\#box

更好写法：\#box

解释：因为id选择器是唯一的，加上div反而增加不必要的 CSS 匹配，这样就会显得多此一举了。

2、不要在 class 选择器前使用标签名

一般写法：span.red

更好写法：.red

解释：同上，但如果你定义了多个.red，而且在不同的元素下是样式不一样，则不能去掉，比如你css文件中定义如下：

p.red{
       color:red;}
    　　span.red{
       color:#ff00ff}

如果是这样定义的就不要去掉，去掉后就会混淆，不过建议最好不要这样写