CSS Tutorial
https://www.w3schools.com/css/default.asp
CSS is the language we use to style an HTML document. CSS describes how HTML elements should be displayed. This tutorial will teach you CSS from basic to advanced.
CSS - Wikipedia
https://en.wikipedia.org/wiki/CSS
Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML.