Manual:$wgResponsiveImages

Details
Whether or not to output high-resolution images for high-DPI displays.

This is primarily targeted at tablet browsers and newer desktops and laptops, such as the MacBook Pro with Retina display. On these high-DPI devices, raster images appear blurry or pixelated by default without special treatment.

When generating thumbnail images, thumbnails are rendered not only at the requested size, but at 1.5x and 2x the requested size. The HTML5 attribute is included on the  specifying these larger images for the appropriate display densities.

Native support for the  attribute was first [ https://www.webkit.org/blog/2910/improved-support-for-high-resolution-displays-with-the-srcset-image-attribute/ added to WebKit browsers] in August 2013 and has since been adopted by most modern browsers, now (as of 2020) reaching over [ https://caniuse.com/#feat=srcset 94% of users globally].

Known limitations
This code applies only to images created through Linker:
 * article thumbnails (since 1.21)
 * file: page detail displays (since 1.23)
 * image galleries (since 1.25)
 * the imageinfo API (since 1.36)

Display densities
Currently (as of 1.39) the densities used (1.5x and 2x) are hardcoded and not configurable. 1x is the implied default density of the main image.