Having spent a lot of time making a similar tool this is pretty difficult. Two issues: (1) highlighting and annotating across (partial) XML with rendered output is very hard without some understanding of the rendering, you can't really just say that a particular highlight is a fixed bounding box or interval; and (2) obviously, web content can change, so even if you could do the above what happens to your annotation highlight from character 5 to 10 when the document shifts?
Personally, I gave up and just converted the web page in question to PDF, and annotated from there. Then you get easy document position markers and no changes over time. I would also be very interested in a system that could do proper webpages natively, though.
Currently, you can create a flashcard and paste a URL in the source field. When you practice the flashcard and "turn it over", if you struggled with the anwer, you can click the source URL and it will open in another browser tab.
We do have plans to release browser extensions that will enable you to create flashcards linked to areas of interest in web pages that you are viewing. We will save the URL and a PDF of the page in case the content subsequently changes.
My biggest desire is to create flash cards directly linked to software documentation.