[gioele]

The technical term for this kind of markup is "stand-off markup" or "stand-off annotation".

There is wealth of research done on stand-off markup since the SGML times, with a renaissance in 2005-2010.

Historically, the main drawback (as highlighted by other comments) is that stand-off needs a way to refer to the content, i.e. a pointing mechanism. Pointers in a pointing mechanism can be either a) hard to write but easy to maintain or b) simple to write but prone to breaking after a change (and thus requiring support from authoring tools). Nobody has found a viable ergonomic balance yet.

[Poefke]

Agreed, you cannot edit one part without the other. But there are many situations where this is not a drawback. Nobody is writing Word documents in a text editor for example.

But I'm looking at a way in which you may be able to add annotations to a document that isn't under your control. If the document is either guaranteed not to change, you won't have a problem. Documents hosted on IPFS or a versioned URL to a Hyper document fit this bill.

If you want annotations to automatically change when a remote document changes, you need to be able to 'follow' the changes. Between when you originally 'linked' to the document and now, what has changed in the document. Which parts are deleted, which parts are added. If you know that, you can update the offsets in your annotations. So I'm looking into making a versioned document format to allow just that.