When an error is detected, suggestions for correcting the issue must be provided – unless doing that would compromise security (like giving a hint to a password for example).
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This helps everyone resolve issues more easily, but especially people with cognitive disabilities who find processing information difficult.
3.3.3 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. (Level AA)
See the W3C’s detailed explanation of this guideline with techniques and examples.
<label for="national-insurance-number"> National Insurance number </label> <span id="national-insurance-number-hint"> It’s on your National Insurance card, benefit letter, payslip or P60. For example, ‘QQ 12 34 56 C’. </span> <input id="national-insurance-number" name="national-insurance-number" type="text" aria-describedby="national-insurance-number-hint national-insurance-number-error" /> <span id="national-insurance-number-error"> <span class="visually-hidden">Error:</span> Enter a National Insurance number in the correct format </span>