CSS stands for "Cascading Style Sheets". It is not a programming language, but a computer language used for styling web pages. It was first released in December 1996 and is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript. These three languages form the basis of most current web pages and web site. All three of these languages are understood, interpreted and run by all modern web browsers.

Before CSS, the basis of web pages was often just HTML where HTML was responsible for both: a) the content/structure of the web pages as well as b) the formatting/layout of the web page. CSS seperated these two, ushering in an entirely new and better way to build web pages and web sites. HTML was now only responsible for content/structure and CSS was responsible for formatting/layout.

The use of CSS in web pages and web sites offers many significant advantages to building and maintaining web sites. These include:

  1. Smaller, more readable and faster loading HTML pages.
  2. External CSS files can be cached, further increasing web page load time.
  3. Formatting and layout changes can be made in one file (versus hundreds), making maintenance of web pages and web sites much faster and easier.
  4. Because the formatting/layout of an entire web site can be controlled in one file, standardization and consistency can be more easily maintained.
  5. CSS (now on version CSS3) had added many new, powerful layout and formatting features, not previously available, e.g. alternating table row colors, positioning, media queries, inheritance, etc.

CSS code can be embedded inside an HTML file (inline and internal CSS) or stored in a seperate file (external CSS). These external files are given a .CSS file extension, e.g. style.css. External CSS is the best method to use in most cases.


CSS was first proposed by Håkon Wium Lie on October 10, 1994. Håkon is a Norwegian web pioneer, a standards activist, and the former Chief Technology Officer of Opera Software. He proposed the concept of CSS while working with Tim Berners-Lee (creator of HTML, founder of the Web) and Robert Cailliau at CERN. As an employee at W3C, Håkon developed CSS into a W3C Recommendation with Bert Bos.

He attended Østfold University College, West Georgia College, and MIT Media Lab, receiving an MS in Visual Studies in 1991. On February 17, 2006, he successfully defended his PhD thesis at the University of Oslo.

In 1999, Håkon was named to the MIT Technology Review TR100 as one of the top 100 innovators in the world under the age of 35.