Style sheets (in English “Cascading Style Sheets”, abbreviated CSS) are a language that allows you to manage the presentation of a Web page. CSS is a recommendation from the World Wide Web Consortium (W3C), just like HTML or XML.
Styles are used to define rules applied to one or more HTML documents. These rules relate to the positioning of elements, alignment, fonts, colors, margins and spacing, borders, background images, etc.
The purpose of CSS is to separate the structure of an HTML document and its presentation. Indeed, with HTML, we can define both the structure (the content and the hierarchy between the different parts of a document) and the presentation. But that poses some problems. With the HTML / CSS couple, we can create web pages where the document structure is in the HTML file while the presentation is in a CSS file.
With CSS we can for example define a set of stylistic rules common to all the pages of a website. This facilitates the modification of the presentation of an entire site. CSS also allows you to define different rules for each display medium (a standard browser, a television, a mobile medium, a Braille reader …). CSS also improves the accessibility of web documents.
In addition, CSS adds new functionality compared to HTML in terms of style. Indeed, HTML allows a fairly basic management of the style of documents.