turns out the default for okular is to save to an external file but there's a setting that can be changed to use the format correctly and store annotations within the file, which is universally compatible with other PDF readers. You can't really blame the format for someone using it wrong on purpose, and if you can then I'll just abuse HTML and the fact that I use it wrong will be evidence that it is, in itself, wrong

That's different. Those are a data structure defining annotations that are meant to be stored externally. They're not part of an HTML file like PDF annotations are. They're meant more for live collaborative commenting within a shared online space, not for making private portable annotations like PDF does.

And it's not a Chrome thing. I don't think any browsers support it, do they? It's not really clear there's a need for it, when collaborative editors already handle document annotations in their own ways.