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.
In the Introduction to HTML module we covered what HTML is, and how it is used to mark up documents. These documents will be readable in a web browser. Headings will look larger than regular text, paragraphs break onto a new line and have space between them. Links are colored and underlined to distinguish them from the rest of the text. What you are seeing is the browser’s default styles — very basic styles that the browser applies to HTML to make sure it will be basically readable even if no explicit styling is specified by the author of the page.
However, the web would be a boring place if all websites looked like that. Using CSS you can control exactly how HTML elements look in the browser, presenting your markup using whatever design you like.
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 can be used for very basic document text styling — for example changing the color and size of headings and links. It can be used to create layout — for example turning a single column of text into a layout with a main content area and a sidebar for related information. It can even be used for effects such as animation. Have a look at the links in this paragraph for specific examples.
W3C Standards & drafts
developer.mozilla.org