How would you build a signup form whose validation errors are actually usable, including for screen reader users?

Use real labels tied to inputs, validate on blur and on submit rather than on every keystroke, and put each error message next to its field with aria describedby pointing at it plus aria invalid on the input. On submit, move focus to the first invalid field or to a summary of errors. Keep native constraints and types so mobile keyboards and autofill work, and never signal errors with color alone.

I start native, because most of it comes free: a real label with a for attribute, the right input type so mobile shows the right keyboard, autocomplete attributes so password managers work, and required where it applies. For timing, I validate a field on blur and everything on submit, never on every keystroke, since telling someone their email is invalid while they are typing it is just noise. Once a field has been marked invalid I do re validate as they type, so the error clears as soon as it is fixed. Each message sits right under its input, and the input gets aria invalid true and aria describedby pointing at the message id, which is what makes a screen reader announce the error when focus lands there. On a failed submit I move focus to the first invalid field, or to a short error summary at the top with links to each field if there are several. And errors are never color only; there is text and an icon, because a red border alone means nothing to a lot of people.