Selectors (选择器)
---------------参考MDN中css学习。
首先css选择器有很多，但总体概括起来的话有两种：
- 标签选择器（*是特殊情况），可但标签，也可上下文多标签；
- 属性选择器（id和class都是属性，特殊的属性）；
标签选择器~单标签
指此单个的标签名字，使用时可直接用该标签进行样式操作，例如操作这段html中标签里的值。
<!DOCTYPE html><html> <head> <meta charset="UTF-8"> <title>Sample document</title> <link rel="stylesheet" href="style.css"> </head> <body> <p> <strong>C</strong>ascading <strong>S</strong>tyle <strong>S</strong>heets </p> </body></html>
在css文件中使用它的标签就能对标签里的属性进行样式操作，比如让字母变成红色。
strong { color: red;}
标签选择器~多标签
官方版
| 选择器 | 选择的元素 |
| A E | 任何是元素A的后代元素E (后代节点指A的子节点，子节点的子节点，以此类推) |
| A > E | 任何元素A的子元素 |
| E:first-child | 任何元素的第一个子元素E |
| B + E | 任何元素B的下一个兄弟元素E |
| B ~ E | B元素后面的拥有共同父元素的兄弟元素E |
通俗解释版
- 选择一个祖先的所有子孙节点，例如 div p{…}
- 选择一个父元素的所有直属节点，例如 div > p{…}
- 选择某一个元素紧挨着的兄弟节点，例如 li + li{…}
- 选择某一个元素的所有同胞节点，例如 span ~ a{…}
- 以上各种情况的组合应用（不要组合过于复杂，编码讲求可读性第一）
给大家列举一个比较典型的应用，如下图
上图中的效果应该比较常见，在各个菜单之间加下划线。我之前的实现是：每个li都加一个border-bottom，在把最后一个li的border-bottom去掉。
其实完全没必要这样麻烦，下面一个样式设置即可解决：
ul li+li{ border-top: 1px solid #ccc;} 栗子引用：http://www.cnblogs.com/wangfupeng1988/p/4282954.html
属性选择器~
最首先当然是要拿出两位重量级选手了：class 和 id
（Class selectors）
通过设置元素的 class 属性，可以为元素指定类名。类名由开发者自己指定。 文档中的多个元素可以拥有同一个类名。
在写样式表时，类选择器是以英文句号（.）开头的。
（ID selectors）
通过设置元素的 id 属性为该元素制定ID。ID名由开发者指定。每个ID在文档中必须是唯一的。
在写样式表时，ID选择器是以#开头的。
我们来看一个栗子：
<p class="key" id="principal">
.key { color: green;}#principal { font-weight: bolder;} 上面的p标签同时具有 class 属性和id 属性，
css中id选择器principal必须是唯一的，而class选择器却是可以和其他标签中的key相同，从而达到多个标签同时操作。
你也可以将多个选择器组合起来构成更确定的选择器。比如，选择器.key 选中所有class属性为 key的元素. 选择器 p.key 选中所有class属性为key的<p> 元素。除了class 和 id，你还可以用方括号的形式指定其他属性。比如，选择器 [type='button'] 选中所有 type 属性为 button 的元素
实例: 使用类选择器和ID选择器
- 创建一个HTML文件
- 创建style1.css
<!doctype html><html> <head> <meta charset="UTF-8"> <title>Sample document</title> <link rel="stylesheet" href="style1.css"> </head> <body> <p id="first"> <strong class="carrot">C</strong>ascading <strong class="spinach">S</strong>tyle <strong class="spinach">S</strong>heets </p> <p id="second"> <strong>C</strong>ascading <strong>S</strong>tyle <strong>S</strong>heets </p> </body></html><br />
strong { color: red; }.carrot { color: orange; }.spinach { color: green; }#first { font-style: italic; }
3.保存文件使用浏览器执行
重新组织样式中规则的顺序，你会发现改变这几条规则的顺序不会影响最终效果。
类选择器 .carrot 和.spinach 比标签选择器 strong 拥有更高优先级。
ID 选择器 #first 比类选择器和标签选择器更优先。
结尾：
第一篇先记录这些吧，后面还有伪类和伪元素，个人还不是很懂，所以先学习学习在进行总结。
刚开始写博客，还有很多的不足个人感觉节奏还是有点问题，希望各位海涵。我会慢慢优化，如有遗漏希望各位前辈指教，一定虚心学习 谢谢~
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