Error Suggestion
If an input error is automatically detected and suggestions for correction are known, then the suggestions are provided to the user, unless it would jeopardize the security or purpose of the content.
In plain language
When a form catches a mistake and a likely fix is known, it should suggest that fix to the person — not just say "there's an error" and leave them guessing.
Why it matters
Just knowing something is wrong isn't enough — people need to know how to fix it. This is especially important for people with cognitive disabilities, who may struggle to figure out the correct format or value on their own.
In practice
Fails this criterion
A phone number field rejects an entry and just shows "Error in field," with no explanation of what format is expected.
Meets this criterion
The same field shows "Invalid phone number — please use the format (00) 00000-0000," giving a clear, actionable suggestion.
Test it yourself
Deliberately enter invalid data into a form field. If an error appears, check whether it suggests how to fix the problem, or just says something is wrong without explaining how to correct it.