Why is Visual Difference Testing still so hard?

This post is going to be half rant and half educational. At least that‘s what I'm aiming for. What is VDT? The concept goes by a few names, but is essentially the same regardless of terminology. You might have heard of “Visual Regression Testing” (VRT) or ”Visual Difference Testing” (VDT). It might be called something else in your sphere, but the idea is the same. Test the visual parts of your application so that when/if anything has visually changed, you can do something about it. I like to call it VDT because I think of the practice as hunting for differences between the current state of a UI and the new state. The difference might not be a regression, it might be a purposeful change or even an acknowledged cross-browser difference like focus states on buttons between Chromium and Webkit. The most common practice—and I think maybe the actual only way to do it—is to render the part of your UI you want to test, take a screenshot of that scenario and save it. Then on your next test run,