The Internet is the definitive cross-platform system, and content is presented on such a huge variety of devices that pages should specify the meaning of the information and leave presentation details to site-specific style sheets.
Cascading Style Sheets or CSS, allows you to control the rendering of fonts, colours, margins, typefaces, and all other aspects of style of a web page.
CSS is a simple style sheet that allows authors to attach a style to HTML documents. It uses common publishing terms, which make it easy for professional designers to fully utilise its features. Visual design concerns, such as page layout are addressed separately from logical structure of the content itself.
One of the main benefits of style sheets is to ensure visual continuity as the user navigates through your site. Websites gain 'brand' cohesiveness if all the pages on a site link to the same style sheet.
By referencing an external style sheet you will get preservation benefits of
being able to update the look of your entire site with a single change to your
stylesheet.
Style sheets are cascading, meaning that a site's style sheet can be amalgamated
with a single web page style to create a cascading presentation across the site.
Different Style sheets can be used for different target platforms to display
the same site data, without changing the data or the content the site itself.
Cascading Style Sheets is an important part of W3C's XHTML: The new standard.