File with a CSS extension contains (in form of plain text) designations for style of a web site, its font, colour, size, border, spaces between lines, indents and location of HTML objects. It is the so-called cascade styles spreadsheet, containing global display properties.

CSS files enable to save similar layout and functionalities of HTML pages and to simplify work of web developers by storing web site’s appearances in a single file. File editing can be performed even in a simple text editor, however, it is advised to perform it within a web site creation environment which provides tools that simplify the work.