Phantom is slow and not kept completely up-to-date. A lot of d3 examples (especially animated visualizations) push the envelope so we should try and get a more robust solution.
a nice looking API that could be interesting:
http://www.nightmarejs.org/
probably the best solution: headless chrome
https://docs.google.com/document/d/1a9cdsljKxCSreR2LDE3KErJhfyC0N9y5ju2UYZb8-J4/edit
via https://twitter.com/samccone