Manual:$wgResponsiveImages

Details
Whether or not to output high-resolution images for hidpi>Special:MyLanguage/HiDPI display support|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].

For current browsers, compatibility JavaScript is loaded which checks   for most browsers, and a compatibility fallback for IE 10, and applies srcset -> src change automatically. On browsers with native   support, this disables itself automatically.

Known limitations
Currently (as of 1.21) this code applies only to images created through Linker methods; image galleries and file: page detail displays are not currently affected. This should be changed at some point by migrating support lower down in the MediaHandler arena.

Display densities
Currently (as of 1.21) the densities used (1.5x and 2x) are hardcoded and not configurable. 1x is the implied default density of the main image.