My time spent with Javascript applications has thus far been pretty brief (working on some aircraft cabin interfaces for a while), but a lot of the time ended up being on testing on numerous different types and sizes of devices, and making tiny tweaks to the CSS to account for as many devices as possible.

This has been a while; perhaps the latest frameworks account for all of that better than they used to. But at that time, I could absolutely see budgeting several days to do what seems like a few hours of work, because of all of the testing and revision.