What is CSS?
什么是CSS?

Styles Solve a Common Problem
用样式来解决一个共同的问题

HTML tags were originally designed to define the content of a document. They were supposed to say "This is a header", "This is a paragraph", "This is a table", by using tags like <h1>, <p>, <table>, and so on. The layout of the document was supposed to be taken care of by the browser, without using any formatting tags.
HTML标签起初被设计成为定义文档的内容。通过使用像<h1>,<p>,<table>这样的标签他们应该表达的是"这是一个标题","这是一个段落","这是一张表格",而布局该由浏览器来处理并非使用格式化标签.

As the two major browsers - Netscape and Internet Explorer - continued to add new HTML tags and attributes (like the <font> tag and the color attribute) to the original HTML specification, it became more and more difficult to create Web sites where the content of HTML documents was clearly separated from the document's presentation layout.
作为两大浏览器 - 网景 和 IE - 不断地将新的HTML标签和属性(像<font>标签和颜色属性)添加到原来的HTML规格中,这让建立清晰的HTML文档内容并从形式中分离出来变的越来越困难.

To solve this problem, the World Wide Web Consortium (W3C) - the non profit, standard setting consortium, responsible for standardizing HTML - created STYLES in addition to HTML 4.0.??
为了解决这个难题,W3C 这个非赢利的,建立标准的组织,为HTML4.0增加了样式

All major browsers support Cascading Style Sheets.
所有主流浏览器都支持样式表

Style Sheets Can Save a Lot of Work
样式表可以提高工作效率

Styles sheets define HOW HTML elements are to be displayed, just like the font tag and the color attribute in HTML 3.2. Styles are normally saved in external .css files. External style sheets enable you to change the appearance and layout of all the pages in your Web, just by editing one single CSS document!
样式表定义元素怎样去显示,就像在HTML3.2中字体标签和颜色属性一般保存在一个外部的.css文件一样.外部样式表能够让你改变所有出现在你WEB中的外观和布局,而仅仅通过编辑一个单独的CSS文档.(原理就是一动多变)

CSS is a breakthrough in Web design because it allows developers to control the style and layout of multiple Web pages all at once. As a Web developer you can define a style for each HTML element and apply it to as many Web pages as you want. To make a global change, simply change the style, and all elements in the Web are updated automatically.
CSS是一个在设计领域中的突破,因为它允许开发者一下就能控制多个WEB页的样式和布局.作为译名WEB开发者你可以为每个HTML元素和应用他的每个页面定义一个你想要的样式.来实现全面的改变,简单的改变样式,所有与之相关的元素都会自动更新

Multiple Styles Will Cascade Into One

Style sheets allow style information to be specified in many ways. Styles can be specified inside a single HTML element, inside the <head> element of an HTML page, or in an external CSS file. Even multiple external style sheets can be referenced inside a single HTML document.?
样式表允许样式信息用多种方式来定义.样式可以在一单独的HTML元素中指定,在<head>元素中或在一外部CSS文件中.甚至多个外部样表能集中在一个单一的HTML文档中

Cascading Order

What style will be used when there is more than one style specified for an HTML element?
当多于一种的样式指定一HTML元素时使用怎样的样式呢?

Generally speaking we can say that all the styles will "cascade" into a new "virtual" style sheet by the following rules, where number four has the highest priority:
一般说来所有样式有下面的规则(第四个最有优先性)

  1. Browser default
    浏览器默认
  2. External style sheet
    外部样式表
  3. Internal style sheet (inside the <head> tag)
    内嵌样式表(在<head>标签内)
  4. Inline style (inside an HTML element)
    行内样式(在一HTML元素内)

So, an inline style (inside an HTML element) has the highest priority, which means that it will override a style declared inside the <head> tag, in an external style sheet, or in a browser (a default value).
所以写在HTML元素中的样式有最高的优先权(写在HTML元素内的),它会替代其他形式的样式.