CSS (Cascading Style Sheets) is a language used to describe the presentation layer of documents written in HTML and XML. Thanks to cascading style sheets, a list of directives is established, which determines how a given graphic element is to be displayed by a web browser. Thanks to CSS you can describe any data responsible for the presentation of elements of web documents, e.g. text color and font, margins or position of a given element relative to other parts of the website composition. CSS was created to separate the structural structure of the document from the form of its presentation. This reduces the complexity of the document, which significantly improves the possibility of making changes to it.