
A typical website is made up of a collection of files (each web page—home page, about page, etc.—is its file). Each file includes HTML code as well as content (text and images). Cascading Style Sheets (CSS) files use to style web pages (CSS). To simplify, consider a standard (non-responsive) website to have a set of files. A few CSS files govern the site’s appearance. A responsive website uses a different set of CSS files based on the device used to view the site.
The site appears and responds differently depending on the device. For example, while a desktop computer displays a horizontally oriented navigation menu, a mobile device displays a vertically aligned navigation menu with a bigger font that is easier to see on a smaller screen.