[Doctor_Fegg]

OpenStreetMap.org's tileservers don't supply retina tiles. But OSM's tileservers are effectively a tech demo and a resource for mappers, not a user-facing service. If you're building a consumer-grade product, you're expected either to use OSM data to render your own tiles, or use a third-party provider which provides tiles based on OSM data (such as Thunderforest, Geofabrik or Mapbox). Leaflet, as the client-side JS part of your mapping solution, can happily show retina tiles from such a provider or your own tileserver.