CSS Libraries
While you can write custom CSS to accomplish nearly any look you want for your webpages, many developers prefer to use a pre-written CSS library that provides rules covering many common user interface needs. For example, CSS-based menus are very challenging to write well, but a well-designed library makes the task very approachable. CSS libraries also provide a consistent UI look and feel across websites, which helps people browsing your website quickly identify how to move around.
One of the most popular CSS frameworks is Bootstrap, which is used by thousands of websites. Other popular CSS libraries include Tailwind, Semantic, and Animate which as the name suggests provides animation components.