What is CSS?

CSS (Cascading Style Sheets) allows you to create great-looking web pages, but how does it work under the hood? This article explains what CSS is, with a simple syntax example, and also covers some key terms about the language.

What is CSS for?

As we have mentioned before, CSS is a language for specifying how documents are presented to users — how they are styled, laid out, etc.

A document is usually a text file structured using a markup language — HTML is the most common markup language, but you may also come across other markup languages such as SVG or XML.

Presenting a document to a user means converting it into a form usable by your audience. Browsers, like Firefox, Chrome, or Edge , are designed to present documents visually, for example, on a computer screen, projector or printer.

### CSS syntax

CSS is a rule-based language — you define rules specifying groups of styles that should be applied to particular elements or groups of elements on your web page. For example “I want the main heading on my page to be shown as large red text.”

### CSS Modules As there are so many things that you could style using CSS, the language is broken down into modules. You’ll see reference to these modules as you explore MDN and many of the documentation pages are organized around a particular module. For example, you could take a look at the MDN reference to the Backgrounds and Borders module to find out what its purpose is, and what different properties and other features it contains. You will also find links to the CSS Specification that defines the technology (see below).

At this stage you don’t need to worry too much about how CSS is structured, however it can make it easier to find information if, for example, you are aware that a certain property is likely to be found among other similar things and are therefore probably in the same specification.