What is Traceur? It doesn't claim to be an implementation of ECMAScript Harmony, though it looks similar to one. The choice of a non-ECMAScript media type (text/traceur instead of application/ecmascript;version=harmony or something similar) lends me to believe that they want to create a new language entirely. Also, text/traceur doesn't isn't registered, so the private type, text/x-traceur should be used instead. It's unfortunate that library authors completely disregard media type registration nowadays. In order for text/traceur to be registered, there actually has to be a formal specification of the Traceur format, since it's on the standards tree.
It's great to see work in making Harmony more accessible to everyone, but it doesn't make sense to make it another language.