It's an HTML, CSS, and JS framework for designing websites and web applications that can be used by every user, for any device, and for any size project. Web developers can use it as a great starting point.
Bootstrap developed at Twitter in 2010 by Mark Otto and Jacob Thornton, and was named "Twitter Blueprint". After more than ten years since its lunch, millions of sites on the web are being powered by Bootstrap.
Bootstrap CDNs (Free):
- https://www.bootstrapcdn.com/ - Powered by StackPath
- https://cdnjs.com/libraries/twitter-bootstrap - Powered by CDNjs
- http://www.jsdelivr.com/projects/bootstrap - Powered by jsDelivr
Note: for full list of free CDN's providers for your projects: Free CDNs.