And he loses his page state if there is an error. You have to do server-side validation regardless, but client-side validation can be a lot more pleasant for the user.

I used to think it doubled your workload to do both, but if you are using JS-free client-side validation, I think that the server can just return an HTTP error guilt-free for any invalid input that the browser will catch. It’s a pretty good compromise for format validation.

Assuming the actual validation of form input takes ~1ms, this is a fairly unusual amount of latency to experience in 2024.

You've got tech stacks and cloud services that will put you within 20ms of 99% of your users.

Server side everything is still the safest possible bet you can make.

The best solution is to use HTMX (or similar) to hook into your server-side validation so that you only have validation code one place (on the server), but you don't have to wait for complete form submission before finding an error in a field.

Example: https://htmx.org/examples/inline-validation/