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Happy paying customer here. A good bookmark manager is an invaluable tool; I use it in conjunction with my personal wiki as a KB tool.

Does Raindrop work offline yet? Can you read a previously-downloaded article, add highlights and annotations, then file it away – without Internet?

Doesn't appear to work offline yet, but their Pro plan has something called a Permanent Copy [1] which sounds like an archived version of a website.

(Just started using it again and asked the same question)

No, I don't think that's possible.