Insufficient contrast between text and background is one of the issues that appears most often in accessibility scans, and at the same time one of the easiest mistakes to prevent. It goes wrong when light grey text appears on a white background, when buttons are only visible through a subtle color change, or when placeholder text in a form field serves as the only indication of what must be filled in. For people with a visual impairment, but also for anyone using a screen in bright sunlight, the text becomes literally unreadable.
WCAG 2.1, the standard that EN 301 549 refers to, requires for level AA a minimum contrast ratio between text and the background on which it appears: typically 4.5 to 1 for normal text and 3 to 1 for large text and for operable elements such as buttons and form borders. This sounds technical, but the underlying principle is simple: contrast is not an aesthetic choice, it is a condition for conveying information.
Where it goes wrong in practice
Contrast problems rarely arise intentionally. They creep in through brand guidelines that look fine on a large, sharp screen in a design studio but fall just short of the standard on an average screen for an average user. Light grey button text on a pastel-colored background is a classic example. The same applies to links that are only recognizable by a color difference, without underlining or other visual indicator, which means people with color blindness simply don't see them as clickable.
Another recurring pattern is the hover state: a button that meets the standard in its default state but changes to a color combination that has not been tested when the mouse moves over it. Errors in forms are also a common mistake: red text on a light red background looks consistent with the brand style, but often fails to meet the required ratio. Anyone curious about what these types of issues look like in their own environment can see in just a few questions where your site stands without having to create an account first.
Why an automated scan solves this partly, but not completely
Contrast ratios are easy to measure automatically: a scanner reads the color values of text and background and calculates the ratio. This makes it one of the few WCAG criteria that a tool can reliably test. What a scan doesn't see is contrast that only emerges after an interaction, such as a dropdown menu, a tooltip that appears on focus, or a status that only becomes visible after a user action. That is exactly why each report states which part has been tested automatically and which part requires manual assessment; contrast is a strong point of automated testing, but not the whole picture.
How to prevent it structurally
The most common way to prevent this problem permanently is to establish contrast requirements at the level of the design system rather than per page. When a brand's base colors are already tested against the standard at the time they are defined, editors or developers don't need to check this repeatedly. This also prevents a new page, a temporary promotional banner, or an externally purchased widget from inadvertently undermining the contrast ratio.
It is also advisable to include contrast explicitly in every visual change to the site, not only at the initial delivery. A house style update, a new seasonal theme or a modified button style can introduce a problem that was not present in the previous version. Accessibility is in that sense not a box you tick once, but a point of attention that runs alongside maintenance. Read more about how such ongoing monitoring works in practice, an explanation of where the assessment comes from.
Finally, it is worthwhile not only to look at the minimum standard, but at the intent behind it: can everyone, even in less than ideal lighting conditions or on an older screen, actually distinguish the text and the buttons. Frequently asked edge cases, such as whether a logo or a decorative image must also meet the contrast requirement, are worked out on the page with answers to questions that come up more often.
Do you want to know whether contrast, and the other recurring issues, also occur in your own environment? The free quick-scan provides an initial indication, and the knowledge base with topics sorted by situation shows how this issue relates to the rest of the obligations surrounding the European Accessibility Act.