Bootstrap is the most famous framework on CSS. You can learn two versions of Bootstrap here. Which are Bootstrap 4 and Bootstrap 5.
Bootstrap 4
Bootstrap 4 is a newer version of Bootstrap; with new components, faster stylesheet and more responsiveness. However, Internet Explorer 9 and down is not supported.
Bootstrap 5
Bootstrap 5 is the newest version of Bootstrap; with a smooth overhaul. However, Internet Explorer 11 and down is not supported, and jQuery is replaced with vanilla JavaScript.
