I've noticed a few bugs with Okular's support of PDFs.

The highlight annotations aren't always recognized on other PDF readers. Specifically, Acrobat Reader, Preview on Mac OS, FoxIt Reader, and PDF.js.

It's probably easiest do demonstrate with PDF.js.

How aggressively is the annotation support being worked on?

I'm on 1.33 and I can try to compile 1.4 but it looks to be a bit of a pain to jump right in

OK. I confirmed with the latest / head unstable Okular that these annotations don't work outside of Okular.

They're broken.

PDF.js doesn't see any highlights..

FoxitReader doesn't see most of them. Usually the most recent ones are missing.

I would say that in the current form that the annotations are broken because the annotation format is standard and should work with other PDF files.

The good news is that Okular itself seems to see them just fine. So maybe broken annotations in Okular can be fixed by re-saving them out.