Field Guide 11: 28 rules for form error messages, from the words to the markup, traced to WCAG 3.3.1, 3.3.3 and 4.1.3 and the GOV.UK error message, error summary and validation patterns. The kit adds a message template by kind of error, a JSON catalogue for an Invite teammate form (16 messages, 0 lint errors), a lint that flags vague, blaming and jargon messages (30 errors across 12 real-world examples), reference HTML and React components, and a Playwright check that submits the form and reads the result (35 of 35 on the reference form, 4 of 35 on the common broken pattern).
When error messages say "Invalid input", when screen-reader users cannot tell which field failed, or before building the form components of a design system.