Maps shown via leaflet at maps.wikimedia.org don't respect devicePixelRatio, so always show low-resolution imagery that looks very blurry on a hi-DPI / 'Retina' display. It appears that higher-resolution imagery is available but not used.
One of the tiles: https://maps.wikimedia.org/osm-intl/16/10436/23444.png
Same tile rendered at 2x density, which should be used instead: https://firstname.lastname@example.org
Note that there seem to be only a few specific density levels supported. If I try an arbitrary @4x, I get this error message back from the tile server:
"This scaling is not allowed for this source. Allowed: 1.3,1.5,2,2.6,3"
So the client-side code will need to either hardcode this list or be able to query it from some kind of configuration or API, then bracket the actual DPR value to the available ones.