What the standard says
If an input error is automatically detected, the item that is in error is identified and the error is described to the user in text.
Why it matters
A red border is invisible to a screen reader and to anyone who cannot distinguish the colour.
What a failure looks like
- failsA form that only turns the invalid field red.
- failsA single "There was an error" banner with no field named.
- failsAn error message not linked to the field with `aria-describedby`.
How to fix it
- doName the field and the problem: "Postcode: use the format SW1A 1AA".
- doLink the message to the input with `aria-describedby`, and set `aria-invalid="true"`.
- doPut a summary at the top of the form with links to each bad field.
How A11ySignal checks it
manual
No automated rule can judge this criterion, and we will not pretend otherwise. A clean scan does not mean 3.3.1 passes — it means nothing we can measure fails. This one needs a person.