css教程:css指令,兼容,注释,selector

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（本文来源于图老师网站，更多请访问https://www.tulaoshi.com/webkaifa/）link rel="stylesheet" type="text/css" href="sheet1.css" media="all" /link标记的用意是允许将html与其他文档相关联。Css用link将css文档与html文档想关联。Css文档虽然不是html的一部分，但是被html使用，从外部style sheets引入它。Link在head元素内，但是不能放在任意head子元素的内部，比如title。Css文档的后缀名虽然不要求，但是有些浏览器不能识别非.css的文件。Link的属性：    rel：代表relation，设为stylesheet。    type：描述数据的类型，设为text/css，告诉浏览器style sheet是css格式的。以后还会有其他的style sheet，比如xsl。    href：style sheet的url。    Media：指定style sheet的使用范围。下列大多数值还不被任何浏览器支持，常用的是all，print，screen。Opera支持projection。可以为media指定多个值，比如media="screen, projection"all Use in all presentational media. aural Use in speech synthesizers, screen readers, and other audio renderings of the document. braille Use when rendering the document with a Braille device. embossed Use when printing with a Braille printing device. handheld Use on handheld devices like personal digital assistants or web-enabled cell phones. print Use when printing the document for sighted users and also when displaying a "print preview" of the document. projection Use in a projection medium, such as a digital projector used to present a slideshow when delivering a speech. screen Use when presenting the document in a screen medium like a desktop computer monitor. All web browsers running on such systems are screen-medium user agents. tty Use when delivering the document in a fixed-pitch environment like teletype printers. tv Use when the document is being presented on a television.    Title：利用title定义多个css文档相互替换的关系。       比如存在如下定义：    link rel="stylesheet" type="text/css" href="sheet1.css" title="Default" / link rel="alternate stylesheet" type="text/css" href="bigtext.css" title="Big Text" / link rel="alternate stylesheet" type="text/css" href="zany.css" title="Crazy colors!" /那么能同时支持多个css定义的浏览器中会有如下表现：       还可以通过将title设定为相同的value来分组：

link rel="stylesheet" type="text/css"

   href="sheet1.css" title="Default" media="screen" /

link rel="stylesheet" type="text/css"

   href="print-sheet1.css" title="Default" media="print" /

link rel="alternate stylesheet" type="text/css"

   href="bigtext.css" title="Big Text" media="screen" /

link rel="alternate stylesheet" type="text/css"

   href="print-bigtext.css" title="Big Text" media="print" /

上面的表述意为：css被title分为两组，default和Big Text。又每一组又被分为print和screen。

 如果有多个link元素，那么只有rel等于stylesheet的link可用。如果可用的link有多个，就会将它们同时作用于html文档，如下：link rel="stylesheet" type="text/css" href="basic.css" / link rel="stylesheet" type="text/css" href="splash.css" / style是引入style sheet最通用的方式。style type="text/css"type：style总是使用type属性，当使用css时，type的值是text/css。Media：与link中一样。 style以style type="text/css"开头，以/style结束，中间是多个styles。这些styles或者指向style sheet文档，或者以内嵌的方式表达。Style元素可以包含多个styles，也可以通过@import指令引入多个指向外部style sheet的链接。用法：

style type="text/css"

 @import url(styles.css); /* @import comes first */@import url(blueworld.css);@import url(zany.css); h1 {color: gray;} /style可见其作用类似link，l         通知浏览器将外部style sheet载入。l         并且可以载入多个style sheet。区别是l         位置与语法不同。@import被包含在style元素中，并且必须在其他css规则之前。l         每一个import的style sheet都会被使用，没有替代规则。相对于link的media属性，import有：@import url(sheet2.css) all; @import url(blueworld.css) screen; @import url(zany.css) projection, print;@import的重要用途：在导入的某个style sheet A中，A需要也使用外部的style sheet，这时link元素显然无用。比如css文档中，是不可能出现link元素的，这时使用@import，如下：@import url(http://example.org/library/layout.css); @import url(basic-text.css); @import url(printer.css) print; body {color: red;} h1 {color: blue;}浏览器对不能识别的tag一律忽略。但是如果浏览器不能识别style元素，style会以普通文本的形式出现在网页的最上面。解决方案：在style里面加上注释符号，这样旧版本的浏览器不会以文本方式显示，新版本浏览器可以正确使用style元素。具体如下：style type="text/css"!-- @import url(sheet2.css); h1 {color: maroon;} body {background: yellow;} --/stylecss的注释类似c：/* This is a CSS1 comment */Comments can span multiple lines, just as in C++:/* This is a CSS1 comment, and it can be several lines long without any problem whatsoever. */但是注意：css的注释不能被嵌套。将style放到html元素描述的地方，就是inline stylep style="color: gray;"The most wonderful of all breakfast foods is  the waffle--a ridged and cratered slab of home-cooked, fluffy goodness... /p这个style属性是一个新属性，可以用到出现body元素中的所有元素上。可以看到style的值是一个字符串，使用和css一样的语法。但是这个字符串只能是一个风格声明块declaration block。不能将@import和css规则放到这个字符串中。就是说只能放css文档中出现在花括号中的文本。 注意：inline style不被推荐使用，在xhtml1.1中inline style是反对的deprecated。因为，它显示违背数据和显示分离的原则。这个原则也是使用css的原因。css核心的特点是将规则应用到元素集上的能力。 Css2规范种关于selector的部分，http://www.w3.org/TR/REC-CSS2/selector.html  css的模式匹配pattern matching规则（css规范，地址如上）： Pattern Meaning Described in section * Matches any element. Universal selector E Matches any E element (i.e., an element of type E). Type selectors E F Matches any F element that is a descendant of an E element. Descendant selectors E F Matches any F element that is a child of an element E. Child selectors E:first-child Matches element E when E is the first child of its parent. The :first-child pseudo-class E:link
E:visited Matches element E if E is the source anchor of a hyperlink of which the target is not yet visited (:link) or already visited (:visited). The link pseudo-classes E:active
E:hover
E:focus Matches E during certain user actions. The dynamic pseudo-classesE:lang(c) Matches element of type E if it is in (human) language c (the document language specifies how language is determined). The :lang() pseudo-classE + F Matches any F element immediately preceded by an element E. Adjacent selectorsE[foo] Matches any E element with the "foo" attribute set (whatever the value). Attribute selectorsE[foo="warning"] Matches any E element whose "foo" attribute value is exactly equal to "warning". Attribute selectorsE[foo~="warning"] Matches any E element whose "foo" attribute value is a list of space-separated values, one of which is exactly equal to "warning". Attribute selectorsE[lang|="en"] Matches any E element whose "lang" attribute has a hyphen-separated list of values beginning (from the left) with "en". Attribute selectorsDIV.warning HTML only. The same as DIV[class~="warning"]. Class selectors E#myid Matches any E element ID equal to "myid". ID selectors