Why is an error message in my checkout form unclear for everyone?

An error message often doesn't say what's wrong and where

An error message is far from clear to every customer because it often uses only colour to indicate that something is wrong, and doesn't also state in text which field it concerns and what exactly needs to change. A red border around an input field stands out to a customer with good vision who uses a mouse, but means nothing to someone who is colour-blind, and is invisible to someone using a screen reader. That same screen reader often reads only the label of the field when filling in an order form — "email address", "postcode" — without mentioning that there is an error message or what it says. The customer then knows that something has been entered, but not that it has been rejected, and certainly not why.

That problem compounds with a second issue: where the message appears. If the text only shows at the top of the page, while the customer is already at the third or fourth field of the form, someone navigating by keyboard or zoomed in simply won't see that message, because it's off-screen. The solution therefore lies not in a different colour or a thicker border, but in text that belongs with the field itself, that is connected to the field so that a screen reader also reads it out, and that explicitly states what must be done — "postcode must consist of four digits and two letters" instead of just "invalid input". Where exactly this goes wrong with forms is described in more detail in why a screen reader sometimes doesn't read form fields aloud, and how that relates to the rest of the ordering process in testing the entire order with only a keyboard.

Where this follows from: WCAG 2.1

That an error message must not rely only on colour and must be connected to the corresponding field follows from the success criteria of WCAG 2.1 on the use of colour and on error identification — the standard that EN 301 549 comes down to in practice. An automated scan signals some of this type of defect, such as the absence of a programmatic link between field and error message, but whether the text of the message is actually comprehensible to a customer remains something that such a scan cannot assess; more on this is in what an automated accessibility test can and cannot find.