I have found that a hybrid viewport screenshot + textual 'semantic snapshot' approach leads to the best outcomes, though sometimes text-only can be fine if the underlying page is not made of a complete mess of frameworks that would otherwise confuse normal click handlers, etc.

I think using a logical diff to do pass/fail checking is clever, though I wonder if there are failure modes there that may confuse things, such as verifying highly dynamic webpages that change their content even without active user interactions.