Cascading Style Sheets, or CSS, is a W3C standard that provides a way to declare the style of html documents. This includes layout, design and typography.
Styles can be declared three ways. One is through an external CSS file linked to an html page. Another is inline style declaration through the style tag. The third is direct html element style declaration using the style attribute.
These three ways comprise the basic “cascade” of declarations that ultimately affects the style of the html document. The CSS style declaration is composed of the selector and the declaration body. The selector states the rule that will dictate which element or elements of the html document the style will apply to, and the declaration body contains the actual style directives. Style directives can affect the layout, design or typography of the html document.
CSS allows web developers to create the style of web pages separate from the html files which contain the content part of a web page. This separation of style and content offers many advantages, including flexibility of design, re-usability of design declarations, consistency of design, and ease of maintenance. Most modern browsers have a rich support for CSS, although each one may have differing implementations of the CSS standard.