Users highly value page transitions that preserve as much context and happen as quickly as possible, but standard page navigations refresh the entire page (header bars, menus, etc disappear and reappear again), increasing actual and perceived loading time. App shell patterns (as with progressive web apps) address this opportunity—this feature is to provide a trivially-easy-to-implement way to get similar functionality with minimal effort.

Scope for Q4 2018 includes technical design