Now we have a nice side-by-side diff in the 'suggested edits' review pages, and as an option in edit historyoption in edit history, could something similar be made available as an option when editing a post?
It wouldn't need to be interactive - a button to get the side-by-side diff of the current state of your edit, versus the original would do. This would be useful when shuffling lots of whitespace around - a final check that you haven't broken some embedded code.
A workaround at the moment is to submit the edit, then check the side-by-side view in the edit history.