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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I think the details of the exercise should be more clear about what a URL may or may not contain for this exercise.
As said in other comments, the exercise doesn't make it clear that www is the only possible sublevel domain of the test cases.