I would like to add to Bill's remarks and suggest that discussion
around ERR thus far has assumed that a single resource maps to a
single representation. The Accept, Accept-Charset, Accept-Language,
and User-Agent request headers can all be used by the server to
respond with varying representations of the resource identified by the
requested URI. The URI alone may not be sufficient to notify the
server of the problem. For instance, if the application/atom+xml,
Shift_JIS, ja version of http://bla.com/feed is invalid, how does ERR
specify that? It might be possible to send the Content-Type and
Content-Language header values as request headers with ERR but now
we're really redifining the semantics of those headers within this
context.