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Understandable · Level AA · 3.3.4

Error Prevention (Legal, Financial, Data)

For pages that cause a legal commitment, a financial transaction, deletion or modification of stored user data, or a test submission, at least one safeguard has to exist: the action can be reversed afterward, the input gets checked and the user can correct mistakes before it's final, or there's an explicit review-and-confirm step before anything actually happens.

  • Cognitive
  • Motor
The problem, in practice — select the image to enlarge it
Four consequential actions — signing a legal contract, paying $250, deleting an account, and submitting a final exam — compared with no safeguard versus at least one safeguard in place. 'Without error prevention safeguards' shows each action completing the instant its button is pressed, with no way to review, correct, or undo. 'With error prevention safeguards' shows the same four actions each gated behind a review-and-confirm step: a 'Review contract' plus 'Confirm & Sign' pair, a payment review screen with 'Confirm payment', an explicit 'Are you sure?' deletion confirmation, and a review screen before final exam submission. A side panel lists the three allowed safeguard types — reversible, checked and correctable, or review-and-confirm — each illustrated with an icon.

How to recognize it

Look at what happens on submission of anything with real consequences — a non-refundable purchase, deleting a saved account, submitting a graded quiz — and check whether a single click or keystroke finalizes it irreversibly with no review step. A 'Delete account' button that deletes immediately on click, with no confirmation dialog and no way to undo, is the clear failure. This matters for everyone who's more likely to make an accidental submission — people with reading disabilities who might transpose digits in a form (a wrong account number, a wrong date) without noticing, and people with motor disabilities who might trigger a click or keypress they didn't intend — but a mistake here isn't a minor inconvenience the way it is elsewhere, since the consequence (money spent, data gone, a legal commitment made) can't just be corrected on the next attempt.

How to fix it

Pick at least one of three mechanisms for high-consequence actions: make the action reversible after the fact (an 'undo' window, a cancellation period); validate the input and let the user correct errors before anything is finalized; or add an explicit review-and-confirm step that shows exactly what's about to happen and requires a separate, deliberate confirmation before it does. A simple confirmation dialog summarizing the action before committing to it is often the cheapest way to satisfy this for something like account deletion.

Example

Avoid

<button onclick="deleteAccount()">Delete account</button>
<!-- deletes immediately, no confirmation, no undo -->

Prefer

<button onclick="showConfirmDialog()">Delete account</button>
<!-- opens a dialog: "This will permanently delete your account. Confirm?" before anything happens -->

Resources