In reply to Jakob.

WP Accessibility does act as an overlay, but only in very WordPress-specific ways, and they’re only applied if the problem is definitely there. That said, WP Accessibility should be treated as a stopgap; if it’s fixing problems on your site, the ideal path is to fix the problems.

WP Accessibility is definitely not an overlay that should cause accessibility problems (although I won’t claim it’s impossible), but accessibility is best fixed at the source, and there are a lot of things WP Accessibility does that fix problems after the fact, like an overlay.