Bootstrap is a free and open-source CSS framework aimed at responsive, mobile-first front-end web development. It includes HTML, CSS, and (optionally) JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components.  All HTML elements in Bootstrap have basic style definitions. As a result, prose, tables, and form elements appear consistently across web browsers. Furthermore, developers can use CSS classes defined in Bootstrap to further customize the appearance of their content.  

The layout components of Bootstrap are the most visible because they affect the entire web page. Because every other element on the page is placed in it, the basic layout component is called a “Container.” Developers have the option of using a fixed-width container or a fluid-width container.

The latest version of Bootstrap is 5.2.2 but it’s backward compatible.  As with previous versions, version 3 and version 4 are still supported for critical bug fixes but no longer add more functionalities.  Bootstrap version 3 is the most stable version.  Bootstrap version 4 is a newer version with a faster stylesheet and more responsiveness.  And Bootstrap version 5 is the newest version but JQuery is replaced with vanilla JavaScript.

Technically, Bootstrap has 3 major versions running, and developers and designers can use the latest version 5 or the old version 3 (still supported critical bug fixes) or stick to the middle ground version 4.  So the questions must be asked:  Which version to choose and why?

Go from Bootstrap version 3 to version 4

Bootstrap version 3 is the most stable version of the three.  It uses a 4-tier grid system containing XS SM MD and LG.  And it’s using less for CSS files, with pixels as the primary CSS unit.

Bootstrap version 4 is using sass for source CSS files and with RAM as the primary CSS unit.  Also, it has a 5-tier grid system and removing access breakpoints and adding excel.

Furthermore, version 4 CSS floats are dropped and CSS flexbox is adopted.  REM is also adopted as a global CSS unit instead of pixels; the global font size is increased.  The grid system is also improving from the previous version.

Go from Bootstrap version 4 to version 5

When upgrading from Bootstrap version 4 to version 5, Bootstrap version 4 depends on JQuery for its Javascript but with version 5, it replaced JQuery with Pure Vanilla Javascript.  Pure Vanilla Javascript is faster and lighter than JQuery due to the very large Javascript file.

Bootstrap 5 supports the latest stable releases of all major browsers.  By using auto prefix Microsoft Edge is supported with version 5.

Responsive font size is enabled by default in Bootstrap 5, allowing text to scale more naturally across devices and viewport sizes. Bootstrap 5 employs RFS, which stands for responsive font sizes, and is capable of rescaling most CSS properties with unit values such as margin padding, border radius, or even box shadow. The mechanism automatically calculates the appropriate values based on the dimensions of the browser viewport and is compiled into calc CSS function with a mix of ram and viewport units to enable responsive scaling behavior.

Bootstrap 4 lacks custom CSS properties for components and layout options, such as table components, whereas Bootstrap 5 includes custom CSS properties for components and layout options.  There are no vertical spacing classes in Bootstrap 4. Bootstrap 5 includes additional classes for vertical spacing. Bootstrap 4 includes margin and padding utilities, but Bootstrap 5 includes additional utility API. Bootstrap 5 is versatile, allowing you to create your own classes and styles while designing projects with utility API. You can also create your own custom utilities.

Support for responsive font sizes has been added. SVG icon library has been added, and  improved and consistent form elements components that are faster and lighter highly customizable by utility API and additional colors grid system placeholder components improved and enhanced edit

Therefore, going from Bootstrap version 3 to 4 is a big step, but going from version 4 to 5 is a major step because Bootstrap version 5 has many more enhancements and updates to make it easier for front-end development.  It all depends on what’s needed and the requirements for building projects.