[lomkiri]

First of all, I guess you're aware that you can annotate an epub wjhen you're using the calibre's epub reader, annotations are stored inside the epub itself, in the "calibre_bookmarks.txt" file (in the prefs, choose miscellaneous / "Keep a copy of annotations/bookmarks in the e-book file, for easy sharing").

But, of course, they are readable only with the calibre's epub reader itself, not even the editor is understanding the content of the file calibre_bookmarks.txt.

If you want to be able to share the annotations with other people that don't use calibre, you're solution is possible, but it needs to write a plugin that, for example with a button on the toolbar, get the current selection and put it inside a span using the css class you've told (the same plugin would create this class at its first call in the current epub). So it's not a trivial solution.

Edit: Or another one that — using the code that Kovid already wrote in the ebook-viewer — would get the content of calibre_bookmarks.txt and put it inside the code of the epub.