It's a shame that this new API still doesn't support querying leap-second information within web-based JS, in any shape or form. The only solutions are either to store a big list yourself and keep it updated, or download it from someone else's website and trust them to keep it updated and keep their CORS settings permissive. Meanwhile, most OSes are fully aware of leap seconds (or at least, browsers could easily include an updated list), but there's just no way to retrieve them from the inside.