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Offline copy is called a snapshot when using the zotero connector [1]

Tags and Hierarchies are both first class.

Technically it is not a bookmark manager. But it manages bookmarks (and much more) very well.

How does Zotero compare to hypothes.is ? I take it that they're both trying to achieve the same goal of annotating webpages.

No they are not.

hypothes.is does indeed annotate webpages (publicly and privately)

Zotero is really a "Database" of references. those references could be simple bookmarks.

The lasted version added to ability to annotate stored pdfs (but not webpages.)