Cascading Style Sheets

"Cascading Style Sheets (CSS) is a simple mechanism for adding style (e.g. fonts, colors, spacing) to Web documents."
More from w3.org

"Cascading Style Sheets save bandwidth, vastly reducing the size of your files when compared to old-style markup. With styles, your sites load faster. You work faster, too. Styles shave grueling hours of grunt-work off your design workload: one brief CSS document can style an entire domain; and when it's time to redesign, you can execute site-wide changes in minutes instead of days."
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For an example of the power of CSS, take a look at what simply changing the style sheet does to one site CSS Zen Garden