Personally i print everything before going. Itinerary, hotel booking, ESTA/Visa/whatever. I also keep those documents on my phone for convencience, and will ha

Personally i print everything before going. Itinerary, hotel booking, ESTA/Visa/whatever. I also keep those documents on my phone for convencience, and will happily checkin using an app 24 hours before arriving at the airport, as well as check in using Apple Wallet or whatever the Android alternative is, but having the documents in printed form gives a certainty when you're stuck in an airport going through security and the "system" breaks down. I have no idea what the TSA would do if you came to them and their system was down, but i assume a printed ESTA would be good enough.