Success Criterion 2.4.13 - Level AAA
When the keyboard focus indicator is visible, an area of the focus indicator meets all the following:
is at least as large as the area of a 2 CSS pixel thick perimeter of the unfocused component or sub-component, and
has a contrast ratio of at least 3:1 between the same pixels in the focused and unfocused states
The focus indicator is determined by the user agent and cannot be adjusted by the author, or
The focus indicator and the indicator's background color are not modified by the author.
Note: What is perceived as the user interface component or sub-component (to determine enclosure or size) depends on its visual presentation. The visual presentation includes the component's visible content, border, and component-specific background. It does not include shadow and glow effects outside the component's content, background, or border.
Note: Examples of sub-components that may receive a focus indicator are menu items in an opened drop-down menu, or focusable cells in a grid.
Note: Contrast calculations can be based on colors defined within the technology (such as HTML, CSS and SVG). Pixels modified by user agent resolution enhancements and anti-aliasing can be ignored.
The purpose of this Success Criterion is to ensure a keyboard focus indicator is clearly visible and discernible. Focus Appearance is closely related to 2.4.7 Focus Visible and 1.4.11 Non-text Contrast. Focus Visible requires that a visible focus indicator exists while a component has keyboard focus; Focus Appearance defines a minimum level of visibility. Where Non-text Contrast requires a component to have adequate contrast against the background in each of its states, Focus Appearance requires sufficient contrast for the focus indicator itself.