I've used CasperJS, PhantomCSS and PhantomJS for visual regression testing on dozens of Drupal sites and, while not a perfect solution by any means, it's worked

I've used CasperJS, PhantomCSS and PhantomJS for visual regression testing on dozens of Drupal sites and, while not a perfect solution by any means, it's worked pretty well. PhantomCSS seems to be pretty configurable and makes it rather easy to detect breaking changes.