CSS is used by both the authors and readers of web pages to
define colors, fonts, layout, and other aspects of document
presentation. It is designed primarily to enable the
separation of document content (written in HTML or a
similar markup language) from document presentation
(written in CSS). This separation can improve content
accessibility, provide more flexibility and control in the
specification of presentational characteristics, and reduce
complexity and repetition in the structural content. CSS
can also allow the same markup page to be presented in
different styles for different rendering methods, such as
on-screen, in print, by voice (when read out by a
speech-based browser or screen reader) and on
braille-based, tactile devices. Similarly, identical HTML
or XML markup can be displayed in a variety of styles,
'brands', liveries or color schemes by using different CSS.
Source: wikipedia
