It was added as part of a bug fix five years ago: the server was looking at the Content-Type request header instead of Content-Encoding to determine whether th

It was added as part of a bug fix five years ago: the server was looking at the Content-Type request header instead of Content-Encoding to determine whether the incoming payload was compressed. Not sure why the Accept-Encoding response header was added as the same time, but it went undetected since it didn't cause any problems (apart from costing money).