Adam: Whilst we standardise on template languages we need to distinguish between the types of template languages that currently exist in the MediaWiki world e.g. handlebars and hogan https://gerrit.wikimedia.org/r/149360 is in the code review queue and basically makes it mandatory to include a template language file extension (previously it would default to hogan when none was specified). Basically you have to rename all your template files from having a .html extension to .hogan and update the ResourceLoader resource modules. Mobile has already done this as have Flow. I think Zero is the only other extension that uses them and I'm keen not to break it. Next steps would be to get Gabriel's knockoff template language integrated into the Mantle extension and allow extensions to give it a go. That then gives us a clear path to migrating this to core once we've decided on a standard template language.