Operable
WCAG 2.4.13
Focus Appearance
Focus indicators must be at least 2px thick around the perimeter with 3:1 contrast between focused and unfocused states. Level AAA in WCAG 2.2. The criterion that finally defines what 'visible' means quantitatively.
In plain terms
Sets the minimum size and contrast for that keyboard outline, so 'visible' actually means easy to see.
Why this matters
A focus ring that's too thin or too low-contrast is technically there but practically invisible — especially for low-vision keyboard users. This criterion defines the minimum that actually counts as visible.
How to detect
Quick check
Inspect focus indicators: are they at least about 2px thick around the element, with 3:1 contrast against both the component and the page background? Compare the focused and unfocused states.