Web-developers use python to develop web-applications, serving HTML or JSON content to user’s browsers which render it. We test our applications to check that they serve content right, and that they respond to HTTP requests in an expected way. However, this is not all that happens in modern web application. There are lots of moving parts which are executed not on the server, but directly in user’s browser, and they need to be tested too. Best way to check that our app works well in a browser is to test it in a browser, and Python gives you tools to do that conveniently.