Include reading system requirements in metadata

The capabilities of reading systems may vary:
For example, some reading systems may have no graphic capability, others may be able to display graphics on a separate line or page, others may be able to display graphics alongside text on the same line(s).
For example, some reading systems may be web-enabled and have ability to access links to books online. Other reading systems may not have any web capability.

Any such requirements about the reading system requirements should be noted in metadata so the reading system can decide "if it can render all, part, or none of the book, or if it can gracefully fall back and advise the user that some features may not be avaiilable.

Proposal:
Information about requirements to render the book should be included in metadata. This should be based on the types of content in the book, e.g.
- has graphics
- has graphics which occur inline with text
- has (floating) page numbers
- has hyperlinks
- has hyperlinks to external resources (e.g. another book, website etc)
- ... and many more, full list to be decided.