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.