The standards are all stored in the features directory.
You can run a manual test of your development copy of a11y at any time just by running:
bundle exec ./bin/a11y <url>
You can also pick out a specific feature to run:
bundle exec ./bin/a11y <url> -- features/04_language.feature
Or you can short-cut a11y's command-line interface and just run Cucumber directly:
URL=<url> bundle exec cucumber features/04_language.feature
The step definitions behind these features are shallow wrappers that delegate to objects in lib/bbc/a11y/. Those objects are unit-tested to ensure they'll behave as expected when tests pass or fail.
To add new behaviour, test-drive changes to the objects in lib.