Visual CSS regression testing is where you screenshot (part of) one version of a webpage in a browser and compare it against a screenshot of the previous version of the same webpage in the same browser. This prevents unintentional changes from happening due to browser version upgrades or changes in the page itself (e.g. refactoring CSS), because we can diff the two screenshots and flag the new version for review by a human developer if there are any differences. If the difference(s) are intentional, the developer approves the change and marks the new screenshot as the new norm/reference/baseline screenshot. By taking screenshots of the same page in different browsers on different platforms, we can extend this to cross-browser cross-platform CSS regression testing.
With that long explanation now out of the way:
What is the best way to do cross-browser cross-platform CSS regression testing?
I'm trying to see what might be usable for Bootstrap.