What Is CSS?

Have you ever thought about what a web page is? I mean, what it really is? Some people think of a web page as a visual medium—an aesthetically pleasing experience which may or may not contain information that’s of interest to the viewer. Other people think of a web page as a document that may be presented to readers in an aesthetically pleasing way. From a technical point of view, the document interpretation is more appropriate.

When we examine the elements of its construction, a web document can consist of up to three layers—content, presentation, and behavior

The content layer is always present. It comprises the information the author wishes to convey to his or her audience, and is embedded within HTML or XHTML markup that defines its structure and semantics. Most of the content on the Web today is text, but content can also be provided through images, animations, sound, video, and whatever else an author wants to publish.

The behavior layer involves real-time user interaction with the document. This task is normally handled by JavaScript. The interaction can be anything from a trivial validation that ensures a required field is filled in before an order form can be submitted, to sophisticated web applications that work much like ordinary desktop programs.

It’s possible to embed all three layers within the same document, but keeping the separate gives us one valuable advantage: we can modify or replace any of the layers without having to change the others.

Certain versions of HTML and XHTML also contain presentational element types—that is, elements that specify the appearance of the content, rather than structure or semantics. For example, and can be used to control the presentation of text, and

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ill insert a visible rule element. However, as these types of elements embed presentation-layer information within the content layer, they negate any advantage we may have gained by keeping the layers separate.

Cascading Style Sheets, or CSS, is the recommended way to control the presentation layer in a web document. The main advantage of CSS over presentational HTML markup is that the styling can be kept entirely separate from the content. For example, it’s possible to store all the presentational styles for a 10,000-page web