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Web Annotation from W3C
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Hypothesis server : Annotate the web, with anyone, anywhere.
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Notes for an annotation SDK / Hypothesis server | source code | h_notify
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WebMention is a web standard for mentions and conversations across the web, a powerful building block that is used for a growing federated network of comments, likes, reposts, and other rich interactions across the decentralized social web.
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(Memex) : local-first knowledge platform. Curate, annotate and discuss the web. As individuals, teams and communities. code
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Fidus Writer : have a look at how they implemented it.
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Outline is working on it. see working doc & issue
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CodiMD has comments
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Coral : open-source commenting platform
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Commento : a fast, privacy-focused commenting platform | code