最近开始详细钻研CSS有关的知识，参考资料是《CSS那些事儿》。将把在此过程中的收获进行记录，方便以后的学习。
一、CSS简介
1.什么是CSS
CSS全称为Cascading Style Sheet（层叠样式表），是一种不需要编译的标记性语言，用于增强控制网页样式并允许将样式信息与网页内容分离。可以使用如何一种文本编辑器对其进行编辑。
2.CSS的作用
a.修饰页面文本、图片等元素，避免使用不必要的HTML元素。
b.更有效地控制页面结构、页面布局（DIV+CSS）。
c.提高开发和维护效率。
3.CSS的基本结构
selector {property:value;}
例如：p {color:#ff0000;}
二、CSS选择符
1.通配选择符
所谓通配选择符，其实只有一个星号（*）而已，通配选择符一般用来对页面所有元素应用样式。例如：
1 * {2 margin:0;3 padding:0;4 color:#ff0000; 5 }/*将页面中所有元素的内外边距都设为0，字的颜色设置为红色*/ 2.类型选择符
以HTML元素类型作为选择符。例如：
1 p {2 font-size:14px;3 text-decoration:underline;4 color:#ff0000; 5 } 3.类选择符
类（class）在编程语言中经常使用，实现代码的复用和封装。在页面中，可以将一段CSS代码定义成一个类，并为其取名，这样也能实现CSS在页面中的复用，减少样式的定义。例如一个名为myContent的css类如下所示。（注意，css的类定义时要以.开头）
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>css-test</title> <style> .myContent{ font-size: 25px; line-height: 5px; text-decoration:underline; font-weight:bold; color: #f00;; } </style></head><body><p>Hello</p><p>css很强大，<strong>可以控制页面<span>任何元素</span>的样式</strong><strong>dfd</strong></p><p class="myContent">1与世界同步，做一个成功的页面仔</p><p>2让我们看看css多么奇妙吧</p></body></html> 运行效果如下图所示：
4.ID选择符
ID选择符与类选择符很像，但是它是以#为前缀的。例如id为myId的选择符：
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <meta charset="UTF-8"> 5 <title>css-test</title> 6 <style> 7 #myId{ 8 font-size: 25px; 9 line-height: 5px;10 text-decoration:underline;11 font-weight:bold;12 color: #f00;;13 }14 </style>15 </head>16 <body>17 <p>Hello</p>18 <p>css很强大，<strong>可以控制页面<span>任何元素</span>的样式</strong><strong>dfd</strong></p>19 <p>1与世界同步，做一个成功的页面仔</p>20 <p id="myId">2让我们看看css多么奇妙吧</p>21 </body>22 </html> 运行效果图如下：
5.包含选择符
包含选择符也成为后代选择符，作用于某个元素所有的后代（子，孙，重孙...），例如改变p标签内的所有strong标签样式 p strong，两个选择符之前用空格隔开。
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <meta charset="UTF-8"> 5 <title>css-test</title> 6 <style> 7 p strong { 8 color: #f00; 9 font-size: 18px;10 text-decoration: underline;11 }12 </style>13 </head>14 <body>15 <p>Hello</p>16 <p>css很强大，<span>可以控制页面<strong>任何元素</strong>的样式</span><strong>dfd</strong></p>17 <p>1与世界同步，做一个成功的页面仔</p>18 <p>2让我们看看css多么奇妙吧</p>19 </body>20 </html> 运行结果如下图所示，可以看出，p标签内的两个strong标签中的内容样式都发生了改变：
6.子选择符
子选择符的主要作用是定义某个元素子元素的样式（两个选择符之前用>连接），而无法定义子元素以外的对象（如孙，重孙都不可以），这是与包含选择符不同的地方。将5中的例子进行修改p strong变为p > strong，表示选择p内的strong子元素。
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <meta charset="UTF-8"> 5 <title>css-test</title> 6 <style> 7 p > strong { 8 color: #f00; 9 font-size: 18px;10 text-decoration: underline;11 }12 </style>13 </head>14 <body>15 <p>Hello</p>16 <p>css很强大，<span>可以控制页面<strong>任何元素</strong>的样式</span><strong>dfd</strong></p>17 <p>1与世界同步，做一个成功的页面仔</p>18 <p>2让我们看看css多么奇妙吧</p>19 </body>20 </html> 运行结果如下图所示：
这时只有dfd是p的子元素，而任何元素是p的孙元素，所以只有前者的样式发生了改变。
7.相邻选择符
a.相邻选择符的表示形式与子选择符类似，只是将>换成了+，主要匹配同一父级元素下某个元素之后的元素。如定义与p相邻的strong元素 p + strong。
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <meta charset="UTF-8"> 5 <title>css-test</title> 6 <style> 7 p + strong { 8 color: #f00; 9 font-size: 18px;10 text-decoration: underline;11 }12 </head>13 <body>14 <p>Hello</p>15 <p>css很强大，<span>可以控制页面<strong>任何元素</strong>的样式</span><strong>dfd</strong></p>16 <p>1与世界同步，做一个成功的页面仔</p>17 <p>2让我们看看css多么奇妙吧</p>18 <strong>3千万不要因为这一点内容就满足了</strong>19 </body>20 </html> 运行结果如下
如上图所示，p标签内的strong元素的样式并没有改变，只是与p相邻的3千万不要因为这一点内容就满足了样式发生了改变。
b.如果将上述代码中的p + strong 改为 p + strong + strong，且增加strong便签，例如：
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <meta charset="UTF-8"> 5 <title>css-test</title> 6 <style> 7 p + strong + strong { 8 color: #f00; 9 font-size: 18px;10 text-decoration: underline;11 }12 </style>13 </head>14 <body>15 <p>Hello</p>16 <p>css很强大，<span>可以控制页面<strong>任何元素</strong>的样式</span><strong>dfd</strong></p>17 <p>1与世界同步，做一个成功的页面仔</p>18 <p>2让我们看看css多么奇妙吧</p>19 <strong>3千万不要因为这一点内容就满足了</strong>20 <strong>4千万不要因为这一点内容就满足了</strong>21 <strong>5千万不要因为这一点内容就满足了</strong>22 <strong>6千万不要因为这一点内容就满足了</strong>23 </body>24 </html> 则运行结果如下图所示
c.如果将b中的p + strong + strong 换成 strong + strong，则运行结果如下所示。
8.兄弟选择符（css3引入）
E~F{property:value }，选择E元素后面的所有兄弟元素F。例如选择p后面的所有兄弟元素p
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <meta charset="UTF-8"> 5 <title>css-test</title> 6 <style> 7 p ~ p { 8 color: #f00; 9 font-size: 18px;10 text-decoration: underline;11 }12 </style>13 </head>14 <body>15 <p>Hello</p>16 <p>css很强大，<span>可以控制页面<strong>任何元素</strong>的样式</span><strong>dfd</strong></p>17 <p>1与世界同步，做一个成功的页面仔</p>18 <p>2让我们看看css多么奇妙吧</p>19 <strong>3千万不要因为这一点内容就满足了</strong>20 <strong>4千万不要因为这一点内容就满足了</strong>21 <div>22 <p title="css-x" id="x">css x</p>23 </div>24 <p title="css+html">css+<span>shishi</span>html</p>25 <p title="ca css d">ca css d</p>26 </body>27 </html> 运行结果如下图所示
由于
css x
位于div中，与上面的p元素并不是兄弟关系，所有并没有改变样式。9.属性选择符
例如p[title|="css"]，代码如下
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <meta charset="UTF-8"> 5 <title>css-test</title> 6 <style> 7 p[title|="css"]{ 8 font-size: 20px; 9 color: #f00;10 margin:0;11 }12 </style>13 </head>14 <body>15 <p>Hello</p>16 <p>css很强大，<span>可以控制页面<strong>任何元素</strong>的样式</span><strong>dfd</strong></p>17 <p>1与世界同步，做一个成功的页面仔</p>18 <p>2让我们看看css多么奇妙吧</p>19 <p title="css-x" id="x">css-x</p>20 <p title="css+html">css+<span>shishi</span>html</p>21 <p title="ca css d">ca css d</p>22 </body>23 </html> 只有title以css开头，且用-连接的元素样式发生了改变
10.伪类和伪对象
伪类和伪对象是一种特殊的类和对象，它由css自动支持，属css的一种扩展型类和对象，伪类和伪对象的名称不能被用户自定义，使用时只能够按标准格式进行应用。
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