The author here: I agree with @Gupie, having unit tests ensures that you have correctly refactored the code and have not introduced new bugs. Checking the unit tests can also help you better understand what problems the code is solving and the edge cases you may be missing.

Normally, if unit tests were missing, I would add them before starting refactoring. In this case, to be honest, I'm not sure how I could have written a unit test to catch the issue. An E2E test is the only one that comes to mind. Please let me know if I am missing an easy way to test for a 404 image when offline. Thank you!