CSS (Cascading Style Sheets) is a web design technology that is used to define the appearance and layout of a website. It is a type of style guide that defines how elements should be displayed on a website.

CSS can be used to define various properties, such as the colour, the font, the spacing between elements, the size of images and much more. By using CSS, web designers can standardise the appearance of a website and improve the visual aspect.

A major advantage of CSS is the separation of content and presentation. This means that the content of a website (e.g. text, images, links) is defined separately from the design (e.g. colours, layout). This makes it easy to make changes to the design without affecting the content. CSS also makes it possible to adapt the design for different devices and screen sizes, which is important for the responsiveness of a website.

Overall, CSS makes the work of web designers easier as it provides an efficient way to control and customise the visual appearance of a website. It is a fundamental technology in web design that contributes to the creation of appealing and professional websites.