How to Stub a series of HTTP requests and test the UI of a web application

I have an ELM application which communicates with an HTTP server to fetch data. I would like to test its UI upon different type of API responses. A test would look something like:

1. Start the app
2. Serve to the app some static data
3. Check the UI behavior
4. Serve some other data
5. Check UI again

and so on.

Such a generic use case has probably a lot of solutions - I'm looking for personal recommendation based on your experience. Preferably, I would like to code these tests in Python (Selenium?) and/or JavaScript.

Thank you.