The advantages of Bootstrap CSS are that its easy to use for first time. Anybody with just basic knowledge of HTML and CSS can start using Bootstrap Responsive features for everyday use. Bootstrap’s responsive CSS can be adjusts to phones, tablets, and desktops Mobile first approach. In Bootstrap 3, mobile-first styles are part of the core framework.Browser compatibility Bootstrap is compatible with all modern browsers .
There are two ways to start using Bootstrap on your own web site.A way to upload bootstrap CSS. There is two ways you can do that.One way you can do is download Bootstrap from getbootstrap.com. You can also include Bootstrap from a CDN.
The bootstrap CSS is so popular because it’s easy to work with, it also provides a lot of functionality for the people, it looks pretty good out of the box, gets you started quickly with a solid framework, and the best is that it is free. But there is no generic solution going to be optimized all use cases, so if you compare a generic framework like the Bootstrap CSS to a more specialized framework for a specific use case, the generic framework will almost always look not to custom. That doesn’t mean it’s not useful as a starting point because for many people for lots of reasons.(http://www.w3schools.com/bootstrap/bootstrap_get_started.asp)
Those that are looking for Bootstrap-only templates may be pigeon-holing themselves, but on the other hand, the framework is popular enough that if they’ll be able to find plenty of templates they can easily customize without having to relearn a whole new set of classes every time. For those that are flipping websites quickly, eliminating that overhead increases efficiency.(https://www.quora.com/Why-is-Bootstrap-so-popular)