Bootstrap is a free and open source tool collection for Responsive websites and web applications. This is the most famous, To develop HTML, CSS and JavaScript frameworks responsive, Mobile-First Websites.
It solves many of the problems we once had, One of which is the issue of cross-browser compatibility. Nowadays Websites are suitable for all brands (IE, Firefox and Chrome) And for screens of all sizes (desktops, tablets, phablets and phones).
Thanks to the Bootstrap developers.