There’s another nice trick you can use to significantly improve the look of a rendered HTML diff. Although this doesn’t fully solve the initial problem, it will make a significant difference in the appearance of your rendered HTML diffs.
Side-by-side rendered HTML will make it very difficult for your diff to line up vertically. Vertical alignment is crucial for comparing side-by-side diffs. In order to improve the vertical alignment of a side-by-side diff, you can insert invisible HTML elements in each version of the diff at “checkpoints” where the diff should be vertically aligned. Then you can use a bit of client-side JavaScript to add vertical spacing around checkpoint until the sides line up vertically.
Explained in a little more detail:
If you want to use this technique, run your diff algorithm and insert a bunch of visibility:hidden <span>s or tiny <div>s wherever your side-by-side versions should match up, according to the diff. Then run JavaScript that finds each checkpoint (and its side-by-side neighbor) and adds vertical spacing to the checkpoint that is higher-up (shallower) on the page. Now your rendered HTML diff will be vertically aligned up to that checkpoint, and you can continue repairing vertical alignment down the rest of your side-by-side page.