Without a clear focus indicator, a keyboard-only user has no way of knowing where they are on a page, making it effectively unusable even if every element is technically reachable via Tab. WCAG 2.2 strengthened this requirement with new success criteria: 2.4.11 Focus Not Obscured requires that the focused element isn’t hidden behind sticky headers or other overlapping content, and 2.4.13 Focus Appearance sets minimum size and contrast requirements for the indicator itself.
A common but harmful practice is removing the default browser focus outline with CSS (outline: none) without providing a replacement, which is a frequent, high-impact audit finding.