CSS stands for Cascading Style Sheets. Cascading Style Sheets is basically a set of rules that specify how HTML components on your website should be display. A CSS rule specifies the HTML element that will change, the presentation property of that element that will change, and the value we want to alter for that property. Like HTML, Style Sheets are not a programming language. It’s also not a markup language. Cascading Style Sheets is a language for creating style sheets. Cascading Style Sheets is used to build one-of-a-kind web pages that are cross-browser and device compatible.