Duolingo uses way too many unecessary JS's. To log in and use the general practice, my browser has to allow at least (besides the from main site duolingo.com) scripts for two other addresses, both of them being randomletters.cloudfront.net. To do the courses, though, something else is needed. Now, I don't know if it is Google Adsense, Google Analytics, Google Adwords, Google Adservices, Ajax Google, Google, Zendesk, Mixpanel, Rollbar, Twitter, Facebook.net, a combination of these, or anything else. I get it, most of these must "add to the experience of duolingoling" or whatever, but isn't it best practices to not depend of a hell lot of BS to run? By simplifying things to have the site's core functionality depend on one website, most people will stil adhere to the default settings, while whoever wants to configure their own internet experience will able to rightly do so.