Main tabs for filters ("Difficulty", "Type", ...) do not have a visible indication for when they receive focus

[jchristgit]

As found in #1363, each keyboard-interactable element must have a visible focus indicator. The focus indicators for GitHub, Reddit, YouTube, would be suitable. It is recommended that site-wide, consistent focus indication is used. Recommendations can be found: https://www.w3.org/WAI/WCAG22/Understanding/focus-appearance.html