The business requirements state what is required but do not specify how the deliverable will actually work. So in many projects with a tangible and technically sophisticated deliverable, it is very common to produce additional documentation about the look and feel of the end product.
The business requirements document should accurately, and in detail, describe the purpose of the project. It states what is needed to achieve that goal i.e. what is in-scope, what is out-of-scope, any assumptions that have been made, any constraints that have been imposed and expected timescales.