First thing I thought of after reading your comment was web inspectors which do help a little bit in figuring out stuff like "Why is this line in italics?" by showing you computed styles. But they could go a lot farther. I'd like to see a temporal graph of everything (e.g. HTML, JS, CSS, Virtual DOM, whatever) that had any visual or behavioral impact on an object, and the reasons why things changed in that order, why some changes overrode others, etc.