Typography refresh/Font choice

This document is in development, see history for sources.

Current situation
The status quo in MediaWiki core, as of February 2014, is as follows: for prose, we specify the neutral and non-descript "sans-serif". This results in the following fonts on the default install on these platforms if I've done my homework correctly:
 * MS Windows: Arial (?)
 * Mac: Helvetica
 * Ubuntu/Firefox: DejaVu Sans (presumably other Linux variants are similar)
 * Ubuntu/Chrome: Liberation Sans
 * Android: Roboto
 * iOS: Helvetica (?)

Note that the differences between Firefox and Chrome on Linux seem to stem from Firefox using the OS standard font resolution mechanism, and Chrome having a built-in heuristic that seems to be very heavily biased toward Liberation Sans.

VectorBeta effects
As of 2014-02-03 (MediaWiki 1.23wmf13-14), VectorBeta uses a "Helvetica Neue", Helvetica, Arial, sans-serif font stack, under which we get:
 * MS Windows: Arial (?)
 * Mac: Helvetica Neue
 * Ubuntu/Firefox: Nimbus Sans L (Helvetica substitute)
 * Ubuntu/Chrome: Liberation Sans (Helvetica and Arial substitute)
 * Android: Roboto
 * iOS: Helvetica Neue

This doesn't seem like a satisfying leap forward, given the level of disruption.
 * By our numbers, a plurality of our users are using MS Windows still (and probably a majority of those using the desktop site). They got Arial before, and they get Arial now. The only way they improve their experience is to buy Helvetica Neue or buy a product that includes Helvetica Neue.  Moreover, it's quite possible that MS Windows users will get a crappy experience with Helvetica if they have an old Type 1 version of it installed on their system.
 * It looks like this causes shift from Helvetica and Helvetica Neue on Mac and iOS, which would seem to be to be pretty subtle. How big is the difference on the site?  The available screenshots don't present a noticeable difference.
 * If cross-platform consistency is the goal, this probably misses the mark. In particular, Android would still be using Roboto, which has quite different metrics than the Helvetica/Arial set of fonts.  Additionally, we still end up with a difference between our two most popular Linux browsers, which while not as large as before, still seems unnecessary.

State of the art on fonts
Here is what seems to be a reasonably well-researched article where the author has clearly put a lot of thought into the cross-platform experience, with the added bonus that it proposes use of free (libre) fonts: http://www.grputland.com/2013/11/multiplatform-helvetica-like-font-stack.html

tl;dr: His stack still lists HelveticaNeue as the first font, but proposes Arimo as a web font which may well look better on MS Windows. Arimo ships with ChromeOS.

It is worth having more research on replacements for free and better alternatives to Arial, because it would seem that it's not hard to do better. While it's unlikely that most MS Windows users will install Arimo, it sends a way better message if we can say "to make your Wikipedia reading experience better, download and install the free font Arimo" than it does to say "to make your Wikipedia experience better, please purchase Helvetica Neue for the low low price of $29.95". Furthermore, it may be worth it to try out the web font mechanism, and we might even be able to talk Mozilla and/or Google into shipping a free font or two with the browser so as to get some real install penetration with these fonts.

Conclusion
In general, it feels as though this iteration is centered around only making the experience for Apple products better, while trying not to break the experience on other platforms, which feels like a low bar. It's not entirely clear how much hands-on effort the User Experience team has put into Windows, Android tablets, ChromeOS, or other Linux desktops, or what the team's goals are for those platforms. The fact that much of the rationale for the new design centers around greater use of Helvetica Neue specifically (which is not free, and is only available to a minority of our users) is annoying, and that seems to be where a lot of the frustration from others comes from as well.