[PaulHoule]

Some kinds of validation really do need the round trip. If somebody is choosing a user name on a sign up for you do need to do a database lookup.

If your back end is fast and your HTML is lean, backend requests to validate can complete in less time than any of the 300 javascript, CSS, tracker, font, and other requests that a fashionable modern webapp does for no good reason...

It's true though that many back ends are run on the cheap with slow programming languages and single-thread runtimes like node.js that compete with slow javascript build systems to make people think slow is the new normal.

[emodendroket]

Yeah, obviously if it requires I/O you can’t write it client side, but the argument here seems to be in favor of doing validation only server-side even when it could also be done client-side.