Here is a list of useful JS and CSS resources with CDN that you can include in your sites. Obviously free and with HTTPS protocol.
jQuery
https://cdn.dotmaui.com/libs/js/jquery-3.2.1.min.js
https://code.jquery.com/jquery-3.2.1.min.js
https://cdn.jsdelivr.net/npm/jquery@3.2.1/dist/jquery.min.js
https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js
jQuery UI
https://code.jquery.com/ui/1.12.1/jquery-ui.min.js
jQuery Mobile
https://code.jquery.com/mobile/1.4.5/jquery.mobile-1.4.5.min.js
Bootstrap CSS
https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/css/bootstrap.min.css
https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css
Bootstrap JS
https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/js/bootstrap.min.js
https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js
Font Awesome
https://opensource.keycdn.com/fontawesome/4.7.0/font-awesome.min.css
https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css
Zepto
https://cdn.jsdelivr.net/npm/zepto@1.2.0/dist/zepto.min.js
https://cdnjs.cloudflare.com/ajax/libs/zepto/1.2.0/zepto.min.js
Modernizr
https://cdnjs.cloudflare.com/ajax/libs/modernizr/2.8.3/modernizr.min.js
Underscore.js
https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js
https://cdn.jsdelivr.net/npm/underscore@1.8.3/underscore-min.js