Cascading Style Sheets (CSS) is a rule based language that applies styling to HTML elements. We write CSS rules in elements (<p>, <img>), and modify properties of those elements such as color, background color, width, border thickness, font size, etc.
CSS3 was started in 1998 but it has never been completed. Some parts are still being developed and some components work on some browsers. It published in June 1999. CSS 3 is divided into several separate documents called "modules". Each module adds new capabilities or extends features defined in CSS 2.
Before learning css you must be basic knowledge of html because css is apply on html so it must be required knowledge of html before learning css.