A CSS (Cascading Style Sheet) is a style sheet language used to define the appearance of elements, for example in HTML language. Styling is done using CSS rules, which consist of selectors for selecting a given element or group of elements, and properties with their values, such as font size or colour. CSS is the foundation of web technology, alongside HTML and JavaScript. CSS 3 introduces many long-awaited novelties such as rounded corners, shadows, gradients, transitions, or animations, as well as new layouts such as Flexbox and Grid Layout. Flexbox - The flexible box model includes a set of CSS properties and values for creating responsive column layouts of HTML elements. Grid Layout - The Grid Layout includes appropriate CSS properties, values and functions to create responsive layouts for grids and columns. In order to organise the code in CSS, different ways of naming classes are used, for example, BEM methodology. In order to optimize work with CSS code, the so-called pre-processors are used. These include SASS, which allows for a multi-level nesting of rules, as well as writing calculations, functions, and loops.

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