When you run the tests, the driver takes a screenshot and compares it to the baseline found in the pdiffs folder.
Run through the instructions below, then after you think this is awesome, add your own features. You'll need to run them twice, once to establish a baseline and a second time to do the diff-ing.
Install selenium2 and imagemagick
brew install selenium-server-standalone imagemagick
Start selenium2
selenium-server -p 4444 &
Grab composer dependencies
composer update
Run the tests (will likely fail)
./vendor/bin/behat --format=pretty,html --out=,report.html