What are accessibility overlays and why don't they work?
In short and in plain language
An overlay is a small add-on script. It promises to make a site accessible on its own.
Experts and disability groups warn against it. An overlay does not fix the faults in the code. It can even clash with the tools users rely on.
The law wants the service itself to be accessible. That is why we fix the causes in the code.
Accessibility overlays are JavaScript widgets embedded via a code snippet that promise to make a website accessible automatically and retroactively, often combined with a control panel for font size or contrast. The idea of compliance at the push of a button is tempting, but does not hold up in practice.
Accessibility specialists and disability associations explicitly warn against these tools (Sources: joint statement by the European Disability Forum and IAAP, 2023; Overlay Fact Sheet). They do not fix the structural causes in the source code, frequently collide with users' own assistive technologies, and can thereby make operation even worse. Lawsuits already exist against websites that remained unusable despite an overlay.
The BFSG requires the service itself to be accessible; that takes structural accessibility in the source code. Instead of masking symptoms, we fix the causes, sustainably and verifiably. How we proceed, you can read under services.