Methinks you should read the spec again. Implementations are free to ignore extensions they don't know about.  The spec actually recommends  that if you wish to provide an extension  type such as a "Frumble" that can gracefully devolve into and be displayed as a "Note" you identify it as a [ "Note", "Frumble" ]. Some projects will try and display this. If you do this backward and call it a [ "Frumble", "Note" ] or just call it a "Frumble", and we have no idea what a Frumble represents, we've every right to throw it into the bitbucket.