CSS
Overview
Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS describes how elements should be rendered on screen, on paper, in speech, or on other media.
Official Resource
The World Wide Web Consortium (W3C) is the current maintainer of CSS and here is their website. However, I would list MDN as the official resource, just like HTML and JavaScript,