1996 December 17th
W3C issued official recommendations for language specification, Cascading Style Sheets, level 1 (CSS1). Cascading styles describe how to display elements in documents written in HTML, XHTML, and XML markup languages.
The first proposal of “Cascading HTML Style Sheets” was published in October 1994 by a Norwegian programmer Håkon Wium Lie, who was working at CERN under Tim Berners-Lee at the time.
In the following months, Bert Bos, a Dutch developer, significantly contributed to CSS1 specification development. In 1997, Håkon Wium Lie and Bert Bos published one of the first cascading style books entitled Cascading style sheets: designing for the Web.