The are four options for the editfont:
- browser default
The current default is 'browser default' which is whatever default your browser picks for <textarea>. In most cases this is monospace, but on OSX + Safari/Chrome (WebKit) it is sans-serif. Pretty much nowhere else in MediaWiki do we let font selection fall through to a browser default, and the same reasons for doing this should apply here (UX consistency).
With the introduction of syntax highlighting, which currently only supports monospace, it would also be better if people were always defaulted to monospace, so they don't get a shock when they turn this feature on.
Affected users will still be able to change their default font.