At Circle [1] are doing practically all of our HTML on the client side. Our server exposes an API using Noir, and we had to build a lot of our own stuff on top of that, like authentication. I added coffeescript and hamlcoffee support to dieter [2]. HTML emails are templated in mustache.

[1] https://circleci.com [2] https://github.com/edgecase/dieter