Success Criterion 2.4.13 - Level AAA
Focus Appearance
The keyboard focus indicator needs to be visible and clear. It should be as big as a 2-pixel line around the unfocused part and have a contrast ratio of at least 3:1. Exceptions are when the user agent controls it or when the author doesn't change it. This rule ensures that the focus indicator is easy to see and understand. It's related to making sure focus is visible and that there's enough contrast for non-text items.