Error Suggestion
When a user input error is detected and suggestions for correction are known, the suggestions must be provided unless doing so would jeopardize security.
What it requires
WCAG 2.0 SC 3.3.3 takes 3.3.1 a step further: error messages must include suggestions for fixing the error when known. Not just "Invalid date" but "Invalid date — please use the format MM/DD/YYYY".
Security exemption: password fields generally don't expose what was wrong (would help attackers). Most other fields can and should give specific guidance.
Common Shopify failure
Address form with "Postcode is invalid" and no suggestion — should be "Postcode is invalid — UK postcodes use the format SW1A 1AA". Phone field with "Invalid format" — should suggest "Use international format: +1 555 123 4567".
How to fix it
AccessComply pairs each form-validation error with a per-field suggestion drawn from a known-formats library; falls back to the merchant's manual error message when present.
Primary source: W3C — WCAG 2.0 Understanding 3.3.3