It's an interesting idea, one that probably wouldn't be very difficult to implement. I do wonder, though, about the overhead that would be necessary for parsing each and every document and creating the tags necessary. Also, I wonder how much larger each page would become, thereby increasing the load-time for the pages.
In any case, thank you for your suggestion! I'll put it on my list of new features to think about (which is actually pretty long at this point!) for this site.