Error Identification
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.
In plain language
If a form catches a mistake automatically (like a missing field or invalid email), it must clearly say, in text, which field has the problem and what's wrong with it.
Why it matters
A color change alone (like a red border) is invisible to screen reader users and hard to notice for people with low vision. Without a text description of the error, they submit the form again and again with no idea what needs fixing.
In practice
Fails this criterion
A form rejects an invalid phone number by turning the field's border red, with no text message explaining what's wrong.
Meets this criterion
The same form shows a text message like "Phone number is invalid — please use the format (00) 00000-0000" right next to the field, readable by anyone, including screen readers.
Test it yourself
Deliberately submit a form with an error (leave a required field blank, or enter invalid data). Check whether a text message clearly identifies which field is wrong and describes the problem — not just a color or icon change.