I expect that an ELM Library instance must provide an elmXmlContent or elmJsonContent (or both). Since both these slices are optional, however, a library with neither would still conform to the profile. Consider adding an invariant that requires one or the other (or both) to exist.
 
To make it easier for implementers, I suggest defining the invariant in terms of the contentType rather than slice membership. E.g., something like content.where(contentType = 'application/elm+xml' or contentType = 'application/elm+json').exists() (note: I haven't tested that fhirpath, but hopefully it's close enough to illustrate the idea).
 
There is also an expectation that within each content, the data is either serialized or pointed to via a url, but this is not formally enforced. Consider adding invariants to each slice to require that data or url exists.