Some browsers have different default textarea fonts than others, so there currently is an option for users to simply use whatever that is in lieu of anything specific.
This causes problems for things which normalise fonts or define their own font stacks (such as OOjs UI and various skins), as it becomes no longer possible to maintain consistent fonts throughout the interface when one random part is explicitly not using the usual fonts. If users want a particular font (such as whatever is equivalent to their usual browser), they should just set their preference to that in general (if they want sans serif, they should just set it to that).
The current options become these classes:
Note: default is not just an option, but
is currently was before August 2017 the default setting for the 'editfont' preference.