Re: [fetch] Access to the HTTP trailer (#34)

As @louiscryan mentioned, proxies are free to promote trailers to headers. This is described in http://tools.ietf.org/html/rfc7230#section-4.3

As a result, HTTP trailers cannot be re-purposed as a fetch-specific metadata mechanism, since proxies may turn the trailers into ordinary headers. In my experience, there exists no feature so terrible that some proxy somewhere won't implement it.

The other consequence is that it is not safe to add trailers to a message on the assumption that the browser will ignore them; an intermediary may promote them to headers at which point the browser will process them normally.

My interpretation of these facts is that trailers can never be safely used on the public web for any purpose, and should be actively deprecated.

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