This CSS Reference Guide provides a comprehensive overview of the Cascading Style Sheets (CSS) language, including syntax, selectors, properties, values, and more. It is designed to help web developers and designers quickly and easily find the information they need to create and maintain websites and
From Wikipedia
Cascading Style Sheets (CSS) is a style sheet language used for specifying the presentation and styling of a document written in a markup language, such as HTML or XML. CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript.