The end-to-end validation of application functionalities is done via integration tests. 

Integration tests check that everything is working as expected from an application user point of view. Therefore, controllers and their corresponding views are tested together.
Like unit tests, integration tests are normally put into their own testing projects and you can use a variety of testing frameworks (xUnit, NUnit, or MSTest). You may, however, also need to use a web server automation toolkit for this type of test, an example of which is Selenium. It must be noted that there is a thin line between integration tests and functional tests, which is a term that other developers use interchangeably, but nonetheless, they are different. Functional tests are more involved than integration tests in the sense that they are used for end-to-end testing and cover the functionality in an application. Integration tests are mainly there to see how certain components actually work with different components. In other words, how do they integrate?

Now that we have looked at the general and high-level picture of what an MVC application consists of, let's get some hands-on experience working with it. The best place to start is with views. This is what our users will be seeing; after all, we all know the term customers first, right? Our users may actually prefer to browse our application through a mobile phone instead of using a desktop PC. How are we going to know which device is accessing our application? How do we make our views responsive so that we can cater for different sized browser screens? We'll answer these questions in the next section.