What do you mean by CSS Framework?
A CSS framework consists of the following workings:
• HTML code which helps make up the formation of the pages.
• Typography method.
• CSS to visually change how components appear.
• Receptive media question.
• Cross-browser compatibility fixes.
Though, with Bootstrap 4 they have switched over to the Sass CSS preprocessor. It bases on a 1,170px grid system which is an open layout. The first strength of Bootstrap is its tremendous reputation. Officially, it’s not automatically better than the others on the list, but it provides many more resources than the other four frameworks collective. In short, Bootstrap is everywhere.