I for independent

Each user story should be self-contained. Ideally, each user story should be prioritized or eliminated without considering any other story. For example, seeing the contents of the shopping cart might seem to be dependent on many things. In reality, creating a shopping cart view is pretty much self-contained. Sure, you could argue that it depends on the existence of the add-to-cart feature, but it doesn't. Two developers could work on those two features at the same time, or one of them could be developed first. Although adding things to the cart helps the website, the view cart can be tested without that feature, with prepopulated shopping carts.