CSS is a web styling language was proposed in 1994 to solve some html4 problems. At that time there were some other styling language also there like style sheet for html and JSSS but at the end CSS won in all.W3C made it recommended to use css2 in 1998.
Like we fill colour in a sketch to make it more beautiful like this, css is used to make html design architecture more unique, colourful and beautiful. It is used for describing the look and formatting of a document written in a markup language. It is most often used to style web pages and interfaces written in HTML and XHTML. CSS supported by all browsers.