One of the main goals of TemplateStyles was to help content creators easily adopt layouts for mobile browsers.
Typically Main Pages have very complex layouts, and those layouts tend to break on small screens. Because of this, other extensions like MobileFrontend have had to special case main pages, striping and reformatting content to get them to display properly on mobile devices. This is obviously not ideal, as it would be a much better solution to allow the content creators to control the style in a way that makes sense for their content.
Additionally, we are in the process of migrating all mobile clients to Parsoid parser markup. Instead of using Mobile Frontend we will be using the new Page Content Service which will be based on Parsoid to deliver HTML to each of the mobile clients. We would rather render Main Pages faithfully from the start instead of special casing Main Pages in the same way that Mobile Frontend currently does now.
The Page Content Service will be used to deliver HTML to the Wikipedia Android app, the iOS App, and the Marvin Progressive Web App.
In order to support faithful, responsive display of Main Page content on mobile, we would like to provide whatever support is needed to Main Page maintainers to migrate to TemplateStyles as soon as it possible. End of Q2 (the 2017 calendar year) would be ideal as it will coincide with the Reader Team migration to Parsoid.
As a start, we created this guide to aide migration: