2.1 规则的结构
<rules>::=<selector>
<左括号><declarations><右括号>
<declarations>::=
<declaration>{<SEMICOLON><declaration>}[SEMICO
LON]
<SEMICOLON>::=分号
<declaration>::=<property><COLON>
<value>
<COLON>::=冒号
<value>::=<keyword list>
<keyword list>::=<keyword>{<SPACE><keyword>}
<SPACE>::=空格
通常会用空格做为value之间的分隔符，有个例外：
As we’ve seen, CSS keywords are separated by spaces—except in one instance. In the CSS property font, there is exactly one place where a forward-slash (/) can be used to separate two specific keywords. Here’s an example:
h2 {font: large/150% sans-serif;}
The slash separates the keywords that set the element’s font size and line height. This is the only place the slash is allowed to appear in the font declaration. All of the other keywords allowed for font are separated by spaces.
2.1.1selector
selector: defines which piece of the document will be affected.
Selector通常是html元素，也可能是xml中允许任何元素。
2.1.2 Declarations and Keywords
2．2 Grouping
2.2.1 grouping selectors
将多个元素共用一个style，例子：
/* group 1 */
h1 {color: silver; background: white;}
h2 {color: silver; background: gray;}
h3 {color: white; background: gray;}
h4 {color: silver; background: white;}
b {color: gray; background: white;}
/* group 2 */
h1, h2, h4 {color: silver;}
h2, h3 {background: gray;}
h1, h4, b {background: white;}
h3 {color: white;}
b {color: gray;}
/* group 3 */
h1, h4 {color: silver; background: white;}
h2 {color: silver;}
h3 {color: white;}
h2, h3 {background: gray;}
b {color: gray; background: white;}
2.2.1.1 The universal selector
* {color: red;}
2.2.2 grouping declarations
例子：
h1 {font: 18px Helvetica;}
h1 {color: purple;}
h1 {background: aqua;}
h1 {
font: 18px Helvetica;
color: purple;
background: aqua;
}
If the second semicolon is omitted, however, the user agent will interpret the style sheet as follows:
h1 {
font: 18px Helvetica;
color: purplebackground: aqua;
}
Since background: is not a valid value for color, and also since color can be given only one keyword, a user agent will ignore the color declaration (including the background: aqua part) entirely. It might render h1s as purple text without an aqua background, but more likely, you won’t even get purple
