forms

Forms stay familiar, for trust and for autofill: normal labelled fields; only the page title carries the prompt style.

rules

Labels are always visible, above the field; a placeholder is never the label.
Inputs are 16 px so phones don't zoom. Usernames use autocapitalize="none", autocorrect="off" and spellcheck="false".
An error sits under the field that caused it, in plain words; with several, an alert at the top says how many.
Submit is either ready, or disabled with a visible reason such as "checking you're human…". It is never enabled only to refuse.
A form can't be sent twice: the button shows its loading state until the answer comes.
Every wait has a timeout and ends in a plain error with a way to try again.

2 things need fixing before you can sign up.

username

use letters, digits and _ only

password

too short — use at least 12 characters

I accept the terms

ok: you're human — thanks