My summary of Nick Stenning's slides about open problems on the Web for annotation at iannotate.
- bi-directional links: need to be able to discover when resources are annotated
- annotation of documents, not formats: an annotation of a document in html should apply to the pdf, epub, etc
- annotating dynamic content: content changes on the Web, and annotations need to be able to survive that (Memento, InternetArchive could help here)
- persistent reference: for annotation Cool URIs is not enough. The Web isn't cool enough.
Don't forget: