Hmmm, all I know of Web Annotation off the top of my head is:
I last wrote about them in:
I have no idea what they've been doing recently though or how well their current tech works.
From a quick skimming of their website, it looks like annotating PDFs are handled.
(They were the ones
way back in 2014–2017ish pushing EPUB in the browser, and last I heard they had HTML annotations working too. Also, they were one of the big pushes behind the
W3C Web Annotation standards.)