CSS规则的结构和Grouping、class和id

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（本文来源于图老师网站，更多请访问https://www.tulaoshi.com/webkaifa/）rules::=selector        左括号declarations右括号declarations::= declaration{SEMICOLONdeclaration}[SEMICOLON]SEMICOLON::=分号declaration::=propertyCOLONvalueCOLON::=冒号value::=keyword listkeyword list::=keyword{SPACEkeyword}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 spacesexcept in one instance. In the CSS property , 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 declaration. All of the other keywords allowed for are separated by spaces.  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。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;}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: purple background: aqua; }Since : is not a valid value for , and also since can be given only one keyword, a user agent will ignore the declaration (including the : part) entirely. It might render s as purple text without an aqua background, but more likely, you won't even get purple s. Instead, they'll be the default color (usually black) with no background at all. (The declaration : will still take effect since it was correctly terminated with a semicolon.)就是同时group selector和declarationh1, h2, h3, h4, h5, h6 {color: gray; background: white; padding: 0.5em;  border: 1px solid black;} You've grouped the selectors, so the styles on the right side of the rule will be applied to all the headings listed, and grouping the declarations means that all of the listed styles will be applied to the selectors on the left side of the rule. 最简单的selector是只针对文档元素的element selecoter，还有两种selecotrs：class selectors和id selectors。这两种selector可以独立于文档元素的，即不是于具体某个的文档元素直接关联的。这两种selecoter可以单独使用，也可以和element selector一起使用。但是这两种selector的使用需要配合文档编写的规范性。比如写一个讨论plutonium处理方式的文档，文档由很多段组成，包含很多警告信息，希望将警告的字体置为bold，以突出显示。但是这些警告信息的格式很多，一段文字，列表式，一小节文本等。所以不能通过p {font-weight: bold;}的形式来定义。这样无法从全是文本的整个文档中找到警告信息，并加粗。因此，解决方式：使用class selectors给警告信息的部分加上标记。