A CSS file is a cascading style sheet (CSS) file used to format the contents of a Web page. It contains customized, global properties for how to display HTML elements. The file can define the size, color, font, line spacing, indentation, borders, and location of HTML elements.
CSS files are used to create a similar look and feel across websites. They also are used to reduce the amount of work and HTML code generated by consolidating display properties into a single file.
CSS files are stored in a plain text format.