Hovercard images appear to always be loaded at 1x density; on a 'Retina'/hi-DPI display this leads to the preview images being a bit blurry. This is especially noticeable on diagrams/flags/maps/etc such in as the below screenshot, but is visible on photos as well.
It seems to be any image generated on the server or client (maybe the latter via oojs or oojs ui) via a standard API (e.g. mw.util.getImageElement / file->getImage() should have data attributes and apply them for hdpi support..
Jon -- that's a good concern to have. I'll look into a cleaner high-level solution for JS stuff...
On PHP side, things using the standard wiki methods of image output should already give the 1/1.5/2x densities in srcset automatically, but I do seem to recall that doesn't cover all output cases. (At least early on, galleries and a couple other places didn't trigger the multiple resolutions.)