Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains CSS– and (optionally) JavaScript-based design templates for typographyformsbuttonsnavigation, and other interface components.

As of August 2021, Bootstrap is the tenth most starred project on GitHub, with over 152,000 stars, behind freeCodeCamp (over 328,000 stars), Vue.js framework, React library, TensorFlow and others.

Bootstrap 2

On January 31, 2012, Bootstrap 2 was released, which added built-in support for Glyphicons, several new components, as well as changes to many of the existing components. This version supports responsive web design, meaning the layout of web pages adjusts dynamically, taking into account the characteristics of the device used (whether desktop, tablet, or mobile phone).

Bootstrap 3

On August 19, 2013, Bootstrap 3, was released. It redesigned components to use flat design and a mobile first approach. Bootstrap 3 features new plugin system with namespaced events. Bootstrap 3 dropped Internet Explorer 7 and Firefox 3.6 support, but there is an optional polyfil for these browsers.

Bootstrap 4

Mark Otto announced Bootstrap 4 on October 29, 2014. The first alpha version of Bootstrap 4 was released on August 19, 2015. The first beta version was released on August 10, 2017. Mark suspended work on Bootstrap 3 on September 6, 2016, to free up time to work on Bootstrap 4. Bootstrap 4 was finalized on January 18, 2018.

Significant changes include:

  • Major rewrite of the code
  • Replacing Less with Sass
  • Addition of Reboot, a collection of element-specific CSS changes in a single file, based on Normalize
  • Dropping support for IE8IE9, and iOS 6
  • CSS Flexible Box support
  • Adding navigation customization options
  • Adding responsive spacing and sizing utilities
  • Switching from the pixels unit in CSS to root ems
  • Increasing global font size from 14px to 16px for enhanced readability
  • Dropping the panelthumbnailpager, and well components
  • Dropping the Glyphicons icon font
  • Huge number of utility classes
  • Improved form styling, buttons, drop-down menus, media objects and image classes

Bootstrap 4 supports the latest versions of the Google ChromeFirefoxInternet ExplorerOpera, and Safari (except on Windows). It additionally supports back to IE10 and the latest Firefox Extended Support Release (ESR).

Bootstrap 5

Bootstrap 5 was officially released on May 5, 2021.

Major changes include:

  • New offcanvas menu component
  • Removing dependence on jQuery in favor of vanilla JavaScript
  • Rewriting the grid to support columns placed outside of rows and responsive gutters
  • Migrating the documentation from Jekyll to Hugo
  • Dropping support for IE10 and IE11
  • Moving testing infrastructure from QUnit to Jasmine
  • Adding custom set of SVG icons
  • Adding CSS custom properties
  • Improved API
  • Enhanced grid system
  • Improved customizing docs
  • Updated forms
  • RTL support