What is CSS/CSS3?

CSS (Cascading Style Sheets) is a style sheet language used for describing the presentation of a document written in a markup language like HTML. CSS describes how elements should be rendered on screen, on paper, in speech, or on other media. CSS3 is the latest evolution of the Cascading Style Sheets language and aims at extending CSS2.1. It brings a lot of long-awaited novelties, like rounded corners, shadows, gradients, transitions or animations, as well as new layouts like multi-columns, box-flex or grid layouts.