Do I comply with the law using an accessibility overlay?

An overlay does not make the site compliant with the law

No, an accessibility widget or overlay does not meet the legal requirement. The European Accessibility Act refers to EN 301 549, which in practice amounts to WCAG 2.1 level AA. That standard sets requirements for the underlying page itself: the structure of the HTML, the labels on forms, the alternative text for images, and usability with keyboard only. An overlay places a layer over the existing page without changing the underlying code. An image without meaningful alternative text, or a checkout button that cannot be reached with a keyboard, continues to exist as it is — even if the widget shows a contrast button or a text size menu. Screen readers respond to the underlying structure, and users of assistive technology regularly report that an overlay hinders rather than helps their software.

What a regulatory authority does when a site is only "solved" with an overlay differs by country, but in none of the countries reviewed does the widget itself count as a solution. In the Netherlands, ACM assesses actual accessibility, and the ACM/Motivaction study showed that a large portion of the webshops examined were not digitally accessible. In Germany, MLBF supervises, but there is also a competition law route through which competitors can send a cease-and-desist letter. In France, DGCCRF tests, and disability organisations have already sent formal notices there due to systemic failures. In Belgium, the Economic Inspection assesses per violation. An automated scan shows which part of the requirements are not met, but even a scan does not assess everything — whether an alternative text truly describes what is visible remains a human assessment.

Where this follows from: EN 301 549 and WCAG 2.1

The directive requires that digital services comply with the harmonised standard EN 301 549 V3.2.1, which for a web interface incorporates the success criteria of WCAG 2.1 level AA. Those success criteria are requirements for the content and the code itself — programmatically determinable name and role of every control, text alternatives, keyboard operability — not requirements for what a visitor sees via a separately loaded widget. An overlay does not change the underlying page and therefore cannot meet the success criteria; the standard itself is the measure, not a layer on top of it.