CSS (Cascading Style Sheet) is used in HTML documents. CSS files are used to define how exactly a website is displayed (e.g. color and font style). The advantage of CSS is that it makes it possible to quickly unify the layout of multiple pages by using only one file to change many files.

If the same CSS is used on multiple pages (usually it’s used on the entire website), it should be store in an external file and only be linked inside the individual HTML documents. This requires less storage space and improves the load time.