The transfer specification defines a resource as (from the 2010-02-16 ED)
A Web service that is addressable using an endpoint reference and can be
represented by an XML Information Set. The representation can be retrieved
using the Get operation and can be manipulated using the Put and Delete
operations.

XML Infosets as defined by http://www.w3.org/TR/xml-infoset/ include a variety
of items not well handled by these specifications such as Unexpanded Entity
References, comments, processing instructions, document type declarations, and
so forth.

This definition also requires that all resources be well-formed XML and not
sequences of elements.
Perhaps there ought to be a more restrictive definition of resource, or the
specifications need to be expanded to deal with all of the infoset (XML,
actually) defined information items.