GitHub has had diff for images for years, and there's a web automation testing (think: Selenium) vendor called Aplitools that takes snapshots of web pages and d

GitHub has had diff for images for years, and there's a web automation testing (think: Selenium) vendor called Aplitools that takes snapshots of web pages and diffs those too. I don't know of any non-proprietary tools, but at least the tech is out there.