Cascading Style Sheets (CSS) is a design language for styling HTML documents. In modern websites, HTML controls the structure while CSS controls its presentation. Let's suppose that you have some HTML formatting in numerous pages across your website. Would you want to manually modify 10,000 HTML elements to implement the new formatting? Ideally, there should be one place where you can make a change that modifies that formatting everywhere you need in the website. Technical term for this concept is "modular design". This technology already exists and it is called CSS.