When aiming for optimal accessibility, beyond basic color contrast checks, what specific design consideration for interactive elements is crucial for users navigating solely with a keyboard?

The crucial design consideration for interactive elements, beyond color contrast, when aiming for optimal accessibility for users navigating solely with a keyboard is a clear and perceivable focus indicator. A focus indicator is a visual cue that highlights which interactive element on a webpage or application currently has keyboard focus. Keyboard focus means that the element is currently selected and will be activated or manipulated by keyboard commands like pressing the Enter or Spacebar key. Without a clear focus indicator, a keyboard-only user cannot visually determine which element they are interacting with as they tab through the page, making it impossible to navigate or operate the interface effectively. This visual cue could be an outline, a change in background color, a border, or an underline, which should be distinct and easily noticeable against the element's default appearance and surrounding elements. The Web Content Accessibility Guidelines (WCAG) recommend that the focus indicator have a contrast ratio of at least 3:1 against the adjacent non-focus-indicating background. For example, when a user presses the Tab key to move from a link to a button, the link might lose its focus indicator (e.g., a blue outline disappears), and the button should immediately gain a new, prominent focus indicator (e.g., a thick, high-contrast border appears around it). This ensures the user knows precisely where their next keyboard action will be applied.