1. can definitely be improved. We can test whether the MIME type is registered. If it isn't, raise a `NoMethodError` with a helpful message rather than bubbling up an internal `NameError`. For example: "To respond to a custom format, register it as a MIME type first: http://guides.rubyonrails.org/action_controller_overview.html#restful-downloads. If you meant to respond to a variant like `:tablet` or `:phone`, not a custom format, be sure to nest your variant response *within* a format response: `format.html { |html| html.tablet { ...`"