The intent of this success criterion is to ensure that users are aware that an error has occurred and can determine what is wrong. In the case of an unsuccessful form submission, it is not sufficient to only re-display the form without providing any hint that the submission failed. The error must be indicated in text. Whether or not an error message provides users with sufficient information about the nature of the error, and what they should do to correct it, is covered more specifically by 3.3.3 Error Suggestion.
An "input error" is information provided by the user that is not accepted. This includes:
For example:
If a user enters a value that is too high or too low, and the coding on the page automatically changes that value to fall within the allowed range, the user's error would still need to be described to them as required by the success criterion. Such an error description telling the person of the changed value would meet both this success criterion (Error Identification) and 3.3.3 Error Suggestion.
The identification and description of an error can be combined with programmatic information that user agents or assistive technologies can use to identify an error and provide error information to the user. For example, certain technologies can specify that the user's input must not fall outside a specific range, or that a form field is required. Currently, few technologies support this kind of programmatic information, but this success criterion does not require, nor prevent it.
It is perfectly acceptable to indicate the error in other ways such as through the use of an image, color, or other visual indicator, in addition to the text description.
This criterion does not mandate any particular way in which errors should be displayed. Depending on the situation, it may be more suitable for all errors to be listed at the start or before a form. In other cases, it may be more appropriate to show errors inline, with error messages next to the specific fields that are in error. Errors could also be listed in an alert, or dialog. This criterion does not cover which of these methods should be used - the only requirement is for errors to be presented to users in text or a text alternative.
See also 3.3.3: Error Suggestion.
An airline Web site offers a special promotion on discounted flights. The user is asked to complete a simple form that asks for personal information such as name, address, phone number, seating preference and e-mail address. If any of the fields of the form are either not completed or completed incorrectly, an alert is displayed notifying the user which field or fields were missing or incorrect.
This success criterion does not mean that color or text styles cannot be used to indicate errors. It simply requires that errors also be identified using text.