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HTML 中引入 CSS 的方式

有 4 种方式可以在 HTML 中引入 CSS。其中有 2 种方式是在 HTML 文件中直接添加 CSS 代码,另外两种是引入 外部 CSS 文件。下面我们就来看看这些方式和它们的优缺点。

内联方式

内联方式指的是直接在 HTML 标签中的 style 属性中添加 CSS。

示例:

<div style="background: red"></div>

这通常是个很糟糕的书写方式,它只能改变当前标签的样式,如果想要多个

拥有相同的样式,你不得不重复地为每个
添加相同的样式,如果想要修改一种样式,又不得不修改所有的 style 中的代码。很显然,内联方式引入 CSS 代码会导致 HTML 代码变得冗长,且使得网页难以维护。

嵌入方式

嵌入方式指的是在 HTML 头部中的

示例:

<head>    <style>    .content {        background: red;    }    </style></head>

嵌入方式的 CSS 只对当前的网页有效。因为 CSS 代码是在 HTML 文件中,所以会使得代码比较集中,当我们写模板网页时这通常比较有利。因为查看模板代码的人可以一目了然地查看 HTML 结构和 CSS 样式。因为嵌入的 CSS 只对当前页面有效,所以当多个页面需要引入相同的 CSS 代码时,这样写会导致代码冗余,也不利于维护。

链接方式

链接方式指的是使用 HTML 头部的

标签引入外部的 CSS 文件。

示例:

<head>    <link rel="stylesheet" type="text/css" href="style.css"></head>

这是最常见的也是最推荐的引入 CSS 的方式。使用这种方式,所有的 CSS 代码只存在于单独的 CSS 文件中,所以具有良好的可维护性。并且所有的 CSS 代码只存在于 CSS 文件中,CSS 文件会在第一次加载时引入,以后切换页面时只需加载 HTML 文件即可。

导入方式

导入方式指的是使用 CSS 规则引入外部 CSS 文件。

示例:

<style>    @import url(style.css);</style>

比较链接方式和导入方式

链接方式(下面用 link 代替)和导入方式(下面用 @import 代替)都是引入外部的 CSS 文件的方式,下面我们来比较这两种方式,并且说明为什么不推荐使用 @import。

  • link 属于 HTML,通过 标签中的 href 属性来引入外部文件,而 @import 属于 CSS,所以导入语句应写在 CSS 中,要注意的是导入语句应写在样式表的开头,否则无法正确导入外部文件;

  • @import 是 CSS2.1 才出现的概念,所以如果浏览器版本较低,无法正确导入外部样式文件;

  • 当 HTML 文件被加载时,link 引用的文件会同时被加载,而 @import 引用的文件则会等页面全部下载完毕再被加载;

小结:我们应尽量使用 标签导入外部 CSS 文件,避免或者少用使用其他三种方式。

参考资料:

  • http://www.stevesouders.com/blog/2009/04/09/dont-use-import

  • http://matthewjamestaylor.com/blog/adding-css-to-html-with-link-embed-inline-and-import

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