PID best practices seems like a good place to post information on the Crossref DOI display guidelines. We updated these back in March 2017, and explain why we did so, moving to https and dropping the ‘dx’ from the domain name portion of DOI links:
N.b. DOIs with the ‘dx’ in the domain name portion still resolve, so you’ll still see a lot of these about - publishers are making updates to this, but it takes time!