This document is intended to contain examples possible of persistent identifier strings and their pros and cons.

An example showing different paths to the same resource as an alternative to content negotiation

Why would this be an improvement? There seems to be concern that content negotiation is the equivalent of polysemy (one name, many meanings)
From URI-based Naming Systems for Science

http://purl.org/commons/record/ncbi_gene/24866
denotes an Entrez Gene record "without commitment as to representation" - that is, the record's declarative content independent of whether the record is rendered as XML, ASN, or RDF. The information in the record may change over time as annotations are added and corrections are made, but it will always be about the same "gene" (a term that unfortunately is not defined).
http://purl.org/commons/xml/ncbi_gene/24866
denotes the XML version of the record.
http://purl.org/commons/html/ncbi_gene/24866
denotes a web page presenting information from the record in human-readable form.