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, "Oxygen-Sans", Ubuntu, Cantarell, "Helvetica Neue", Helvetica, Arial, sans-serif;
This is the semantic order of things -
User device preferred > Wikimedia Design preferred > Helvetica > Arial > Generic
 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.