A form looks like the simplest thing on a page and contains more edge cases than the rest of the application.

Timing is most of the experience

Validating on every keystroke tells someone their email is invalid while they are typing the third character. Validating only on submit makes them fix six things at once. The rule that tests well: validate on blur, revalidate on change once a field has already errored, and never move focus without the user asking.

The parts that get skipped

• Errors need to reach a screen reader. A red border is not an announcement; associate the message with aria-describedby and put a summary at the top that links to each field.
• Autofill saves more time than any feature you will build. Use the standard autocomplete tokens and test it, because a custom component usually breaks it silently.
• The back button must not lose what someone typed. This is the complaint users actually voice.
• One column. Two-column forms consistently test worse and the reason is scanning order.