At English Wikisource we are having issues that the default display for users logged in, and logged out is the PNG images setting and that is not helpful for display of transcribed pages.
eg. https://en.wikisource.org/wiki/Page:Radio-active_substances.djvu/49
Gives errors in PNG and sometimes in Latex
Browser statistics show that users are using more modern browsers, so it seems preferable to have the default to the modern display of math, and allow users to rollback to the other preference if they so choose.