What is an accessibility overlay and why do experts warn against them?

An accessibility overlay is a third-party JavaScript widget that scans a page after it loads and tries to auto-inject fixes — like alt text or ARIA labels — without touching the underlying HTML. The core problem is mechanical: automated scanning, which is all an overlay can do, catches only 30-40% of WCAG failures, and the patches it injects can conflict with existing scripts, hijack keyboard focus, and disrupt screen readers that were already working correctly before the overlay loaded. Structural issues — a broken heading hierarchy, an inaccessible custom dropdown, mismatched form labels — live in the source code and simply can't be repaired from the outside. That's a separate problem from the marketing claims the FTC fined accessiBe for; even a completely honest overlay vendor would still run into these technical limits.