CSS is an abbreviation for Cascading Style Sheets. Style sheets are text files (.css) that define how to display HTML elements. They give the designer more control over the webpage layout. CSS lets determine fonts, colors, margins, lines, height, width, image background, object positions and many other things. CSS markup is supported by all web browsers.
Any webpage consists of 3 layers: content, presentation and behavior. The content layer is the information that the author renders to his readers. By content we mean text, sound, video, animations, etc. It is embedded with the help of HTML or XHTML markup into a webpage. The presentation layer defines how the content will be presented to a human being. And finally the behavior level comprises real-time user interaction with a web document.
CSS is the recommended way to convey the presentation level. It was created to separate content from design. The main benefit of CSS markup over similar HTML markup is that we can modify or replace any layer without having to change the others.
Let’s focus on why style sheets are called cascading. Every web page is managed at least by one style. But there may be several styles affecting the appearance of one and the same web page. They are applied in a “cascading” order.
Style sheets can be classified into internal and external. An internal style sheet only affects the HTML document in which it is imbedded. On the contrary, an external style sheet is independent and can be attached or linked to many HTML documents. If external style sheets are used, one can modify the entire appearance of a website by altering just one file.
The main advantages of CSS markup usage:
- layout control of many documents with a single style sheet;
- more precise formatting;
- presents different media in various ways (screen, print, etc.);
- gives a possibility to apply complex design techniques
- easier site maintenance
- web accessibility
Image source: wpthemes.tv