Bootstrap is a free front-end framework with the ability to easily create responsive designs.

Bootstrap includes HTML and CSS based design templates for typography, forms, buttons, tables, 

navigation, modals, image carousels and  many other, as well as optional JavaScript plugins 

Bootstrap is compatible with all modern browsers (Chrome, Firefox, Internet Explorer, Edge, Safari, and Opera)

include it in head 

<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">

<!-- jQuery library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<!-- Latest compiled JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script> 

For faster loading time.

Many users already have downloaded Bootstrap from MaxCDN when visiting another site.

As a result, it will be loaded from cache when they visit your site, which leads to faster loading time.

Also, most CDN's will make sure that once a user requests a file from it, 

it will be served from the server closest to them, which also leads to faster loa11 ding time