Knows the basic principles of web accessibility and what makes for an accessible website. Can test for keyboard accessibility.
Knows how to use at least one screen reader to test websites. Knows all the common accessibility antipatterns (keyboard traps, labels in images, etc) and can identify them on sites. Can use automated tools like AXE to analyze websites and report back to developers on issues. Can check all WCAG AA 2.1 rules which do not involve looking at actual code.
Can check the full WCAG AA 2.1 checklist including reviewing code for invisible or structural problems. Understands common solutions to accessibility issues.
Understands the qualitative experience of disabled users. Has extensively used a screen reader and knows how to use it the way a real-world user would.
Is an expert on web accessibility, knowing the WCAG criteria intimately but also understanding the qualitative experience of disabled users.