CSS stands for Cascading Style Sheets and it is the language used to style the visual presentation of web pages. CSS is the language that tells web browsers how to render the different parts of a web page.
Every item or element on a web page is part of a document written in a markup language. In most cases, HTML is the markup language, but there are other languages in use such as XML. Throughout this guide we will use HTML to demonstrate CSS in action, just keep in mind that the same principles and techniques apply if you’re working with XML or a different markup language.