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You can switch the template engine in Ember, but you'll lose the databinding hotness unless you implement it in your own engine.

Do they provide some kind of spec/API for other engines to support it? That would be a killer feature.

Yeah, still waiting for jade support but haven't looked at how to implement one yet.

You might want to give Emblem a shot. It's similar to HAML or Jade but compiles to Ember/Handlebars-compatible templates: http://emblemjs.com/