The purpose of Cascading Stylesheets (CSS) is to make it easier to format your HTML pages. Styles created with CSS work much like styles in word processing documents. They set named combinations of font characteristics (like size, color, face, and weight) and display properties (like alignment, margins, background, and borders). By changing a style's definition, all elements which use that style are automatically changed, too.

Styles can be defined in a separate linked document, embedded by using STYLE tags in the HEAD of the page, or inline within tags. Only the linked stylesheets allow the global changes that are the power of CSS. Proper syntax is covered under Terms and Syntax

To see examples of stylesheets in action, use the Effects links on the left.
Wondering what it looks like in the "other" browser? Effects (Images) links don't use your browser to produce the effects, but show images instead.

Some pages use the linked stylesheet demo.css which you can view here.

This demo works only with browsers that understand stylesheets, like MSIE3.x+ and MSIE4.x+. NC4.x handles some of this the same, but not all.
To see the source code for a frame, right click inside the frame and choose View Source. Or select from the menu View | Frame Source.