For what purpose? It should be easier/lighter to check if the HTML, CSS or JS has changed if you'd like to detect a defacement or a site load issue (actually, the HTTP status code would be different for one or more resources; 404, 500, etc)

Visual consistency checking is more comprehensive than source code diffs. For example, errors in third party services (a region-blocked ad, a youtube iframe, etc...) may not show up in error logs or HARs.

That sounds like a good idea on paper, but I think rotating or animating content (especially advertising banners) would heavily complicate that strategy.

There are also pop ups (like the EU cookie warning), self-triggering modals, A/B testing experiences, and newer patterns like feedback widgets that give false positives.

I am working on a visual diff engine that will account for all of these, you can sign up for updates here:

http://geodiff.launchrock.com/

Fair point.

I don't deal with a lot of sites that serve ads, but I do deal with a lot of sites that are on CDNs. Was more my target user group.

Thanks dbg31415, I have pushed that fix. That is an excellent feature suggestion.