MediaWiki: 1.23.0
PHP: 5.3.10-1ubuntu3.11 (apache2handler)
MySQL: 5.5.32-0ubuntu0.12.04.1
Example of error: http://www.pidgi.net/wiki/File_talk:Mario_Kart_TV_logo_(alt)_-_Mario_Kart_8.svg
The code speaks for itself. srcset=" 1.5x, 2x"
Nothing is being generated for these, leaving the image as a broken link in browsers that support the property and fit the criteria.