xmlns.com is an internet domain created for the purposes of simple Web namespace management. The rationale for registering xmlns.com was to secure a short, memorable domain suitable for naming concepts for use in RDF and XML vocabularies.
You can tag arbitrary content on the web, you can do it in a low-tech way to make it easy for everyone to do...But...How do you find instances that people haven't tagged? Or deal with overlapping meme labels?