Basic Concepts

In the past, web programmers had to program all appearance information for a page using HTML tags, marking each individual item on a page separately. If someone later decided that they didn't like the way the website looked, the programmer would have to go in and change all of the HTML tags separately, one at a time, a tedious and time consuming task. In addition, the control possible over HTML appearance using only HTML tags was primitive, at best.

CSS has greatly improved this situation. Not only does CSS allow for much greater control over HTML appearance, it also provides a mechanism for making global appearance changes to a group of HTML pages, or even an entire website.

CSS allows you to define what HTML tags will look like, and how they will behave (to some degree). CSS also allows you to create external ".css" files, which contain all of the HTML tag appearance information. These ".css" external style sheet documents can then be applied to as many HTML pages as you like. With a single change made to a ".css" document, you can change the appearance of EVERY HTML page attached to that ".css" document, a great convenience compared to the old methods of changing each individual HTML tag on each page by hand. And CSS gives you a tremendous amount of control over every aspect of HTML tag appearance, from color and spacing to position and behavior.

Because the HTML's appearance characteristics are entirely separate from the HTML itself, you can actually substitute ".css" style sheets on a website whenever you like, changing the look and feel of a website instantaneously, at will.

With tremendous power over HTML tag appearance, and the ability to make changes to that appearance quickly and easily, CSS allows the web programmer great flexibility when designing websites. With this increased control, however, comes a potential for greater complexity, especially in the ".css" documents themselves. A thorough understanding of CSS principles is necessary, therefore, both to create your own CSS-enhanced websites, and to edit the CSS-enhanced websites of others.

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