I found this page recently: http://arcgis.github.io/tailcoat/ but the repo on GitHub, or the link to it, seems to be broken. Is this something that ESRI is planning on making open to the developer community anytime soon? I would be extremely interested in getting my hands on it and moving away from .claro. I see that some of the boilerplate JS viewers available in ArcGIS Online are implementing the calcite.css, and would like to start exploring it. Especially when trying to extend these application templates, to prevent collisions with other stylesheets.
Any info is appreciated.