CSS
CSS is a set of instructions that define how a website should be styled
This stands for Cascading Style Sheet. Like HTML, it is not a full-blown programming language. Instead, it is a list of commands delivered along with a website to a browser and executed by the browser.
CSS is considered client-side i.e. not by PHP, which sits on the server-side. Its sole purpose is to tell the browser how to style the HTML delivered into the browser.
See Also