css 样式由选择符和声明组成，而声明又由属性和值组成：
选择符：
又称选择器，指明网页中要应用样式规则的元素，如本例中是网页中所有的段（p）的文字将变成蓝色，而其他的元素（如ol）不会受到影响。
声明：在英文大括号“｛｝”中的的就是声明，属性和值之间用英文冒号“：”分隔。当有多条声明时，中间可以英文分号“;”分隔，如下所示：
p{font-size:12px;color:red;} 注意：
1、最后一条声明可以没有分号，但是为了以后修改方便，一般也加上分号。
2、为了使用样式更加容易阅读，可以将每条代码写在一个新行内，如下所示：
p{ font-size:12px; color:red;} CSS代码注释
就像在Html的注释一样，在CSS中也有注释语句：用 /*注释语句*/来标明（Html中使用 )
CSS代码插入形式
从CSS 样式代码插入的形式来看基本可以分为以下3种：内联式、嵌入式和外部式三种
内嵌式：
css样式表就是把css代码直接写在现有的HTML标签中，如下面代码：
这里文字是蓝色。
CSS代码需要写在元素的开始标签里。
并且css样式代码要写在双引号中，如果有多条css样式代码设置可以写在一起，中间用分号隔开。如下代码：
这里文字是红色。
嵌入式css样式，就是可以把css样式代码写在标签之间。如下面代码实现把三个标签中的文字设置为红色：
<style type="text/css">span{color:red;}</style> 嵌入式css样式必须写在<style></style>之间，并且一般情况下嵌入式css样式写在<head></head>之间。
外部式：
外部式css样式(也可称为外联式)就是把css代码写一个单独的外部文件中，这个css样式文件以“.css”为扩展名，在
内（不是在代码的前面（实际开发中也是这么写的）总结来说，就是--就近原则（离被设置元素越近优先级别越高）。
注意：上面所总结的优先级是有一个前提：内联式、嵌入式、外部式样式表中css样式是在的相同权值的情况下。(权值是什么玩意。。)
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