update the font-stack for Wikipedia on mobile web which gives preference to system typeface
Apple and Google both have better typeface alternatives available. 
They have better readability on screens and they have better language support.
This will deliver better native experience for readers. 
Proposed stack (see discussion for other links/suggestions)
font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Lato, 'Helvetica Neue', Helvetica, Arial, sans-serif;
This is the semantic order of things -
User system preferred (mobile) > Wikimedia Design preferred (Lato) > User operating system desktop only ( 'Helvetica Neue' > Helvetica > Arial) > generic sans-serif
 a https://fonts.google.com/specimen/Roboto
 b https://material.io/guidelines/style/typography.html
== Developer note
There's a [[ https://booking.design/implementing-system-fonts-on-booking-com-a-lesson-learned-bdc984df627f | known bug for Internet Explorer/Edge ]] with this stack when used in `font` shorthand.
Can be circumvented when used only with `font-family`.
== Open questions
Jan has been doing some excellent research in [[ https://phabricator.wikimedia.org/T175877#3831247 | the comments ]]. We should read through that and decide if we need to make any adjustments before we consider this good to work on.
=== Update Sep 2018
- Removing Linux OS fonts `'Oxygen-Sans', Ubuntu, Cantarell`, following GitHub's findings.
=== Update Nov 2018
- Removing `system-ui` as other platforms have run into issues with certain OS/languages combinations with it. See T175877#4776576
== Prior art
* [Booking.com: Implementing system fonts lessons learned](https://booking.design/implementing-system-fonts-on-booking-com-a-lesson-learned-bdc984df627f) at booking.design (March 2017)
* [Skyscanner: A native font-stack](https://medium.com/skyscanner-design/a-native-font-stack-d9d0db72d6e6) at medium.com (April 2017)
* [A guide to System Font Stack](https://css-tricks.com/snippets/css/system-font-stack/) at CSS-Tricks.com (Sept 2017)
* [Medium.com: Sytem shock](https://medium.design/system-shock-6b1dc6d6596f) at medium.design (Oct 2017)
* [GitHub: Shipping system fonts](http://markdotto.com/2018/02/07/github-system-fonts/) at markdotto.com (Feb 2018)