I myself have been using bootstrap in all my projects since 2013. Boostrap has been the go-to CSS framework for most developers for a number of years, but recently tailwind has been gaining popularity among developers.
CSS is a “utility-first” CSS framework that provides a deep catalog of CSS classes and tools that lets you easily get started styling your website or application. Tailwind CSS provides a wide variety of CSS classes that each have their own role of use. Instead of a class called .btn that is created with a bunch of CSS attributes directly, in Tailwind, you would either apply a bunch of classes like bg-blue-500 py-2 px-4 rounded to the button element or build a .btn class by applying those utility classes to that selector.