While when annotating on paper it's just making physical marks, making 'digital' marks can vary wildly when there's no standard (i.e. when we're not dealing with a PDF). If we believe in maximum interoperability, then we should agree on a 'fallback' standard when the format doesn't offer its own way of saving annotations. HTML is a hugely popular format, and HTML conversion to other formats is often a disaster: ideally researchers, students, developers and the like would annotate directly on HTML, it has many benefits. If Okular implements HTML I can use it for everything everywhere yay (Okular Mobile :) )!
We can not implement HTML in Okular. Okular is a page oriented viewer, HTML does not have the concept of page.
Oh hey, didn't think of that, thanks for pointing that out. In that case, HTML may need their own standard of annotations, based on files rather than pages...