Should `paged`, `facing-pages` and `non-paged` descriptions be clearer?

[tomcrane]

Define "page turning interface"?

[cmahnke]

This would be great. Maybe that's one of the reasons why this is currently not very well implemented, examples:

• viewingHint not fully implemented  tify-iiif-viewer/tify#181
• Support facing-pages and non-paged viewing hints ProjectMirador/mirador#3029