How do I save an HTML document locally, and annotate it, in an easily sharable form, and in a form that is stable - i.e., in a way that will be readable and useable in 20-50 years?
Basically any HTML document from 20-30 years ago (can't go any further because it didn't exist 50 years ago) will be completely readable and usable. The only issue is people creating content (not styling) in formats besides HTML.
As far as annotations, you can use the native <ruby>[1] tag, or strikethough, but if you mean "literally drawing on the text" then, yeah, you're looking for an image format at that point (which is fundamentally what PDF is), but we shouldn't default to storing text in image formats just because of one specific use case. (Also, as I said above, the only reason tools exist to easily do that in PDFs exist is because everyone insists on using a format that's hard to edit. )
Also, note that the context I was responding to was US legal documents, not something more presentation-heavy.
You say it as if pdf is somehow better. To begin with it's a proprietary format. If Adobe goes bankrupt or obscure tomorrow, pdf will go out of use as a failed technology.
As far as annotations, you can use the native <ruby>[1] tag, or strikethough, but if you mean "literally drawing on the text" then, yeah, you're looking for an image format at that point (which is fundamentally what PDF is), but we shouldn't default to storing text in image formats just because of one specific use case. (Also, as I said above, the only reason tools exist to easily do that in PDFs exist is because everyone insists on using a format that's hard to edit. )
Also, note that the context I was responding to was US legal documents, not something more presentation-heavy.
[1]https://twitter.com/antumbral/status/1730829756013375875