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WAI: Strategies, guidelines, resources to make the Web accessible to people with disabilities

[DRAFT] Error prevention

  • What to do: Build in a layer of protection to prevent errors when a form that has significant consequences for users such as financial transactions, anything legally binding, where the user's data may be deleted from a database or where a form is used to submit test responses.
  • Why: Users may spell some important data incorrectly or transpose numbers when typing in their personal information and should have an opportunity to correct errors before important submissions are accepted.
  • How: There are three optional ways that error prevention mechanisms can be put in place:
    • Reversable: Where submissions are reversible, and the user is able to cancel a transaction or undo any recent changes to a database, for instance. . Checked: The data entered by the user is checked for input errors and the user is given an opportunity to make any necessary corrections.
    • Confirmed: Where a mechanism is available for reviewing, confirming, and correcting information before finalizing the submission.
  • Conformance: required to meet SC 3.3.4 Error Prevention (Legal, Financial, Data).


An online shopping Website lets users cancel purchases up to 24 hours after the order is submitted. The Web site explains their policy, and includes a summary of the policy on the purchase receipt emailed to the user. After 24 hours, the purchase will be shipped to the user and can no longer be canceled.


Instant and irreversible transactions, especially on a single page, can provide users with a checkbox to confirm that they do want the requested action to take place. For instance currency conversion transactions , where charges or changes in exchange rates would not permit the transaction to be reversed without cost to the user. After collecting the account and amount data, and before the submit button, a checkbox with a label like "I have checked that the amount I wish to transfer is correct". The form should then fail to validate unless the checkbox is checked.


The checkout page on a shopping website includes a form that collects order, shipping and billing information. After submitting the form, the user is taken to another page where the information they have submitted is summarized for review. Below the summary, The user must activate a "complete order" button on the review page to complete the transaction.