I always thought that websites are tested with Selenium and similar (https://www.seleniumhq.org/ https://www.seleniumhq.org/). For other kind of GUIs, you can

I always thought that websites are tested with Selenium and similar (https://www.seleniumhq.org/ https://www.seleniumhq.org/). For other kind of GUIs, you can also emulate mouse + keyboard interaction with tools like Sikuli (http://www.sikulix.com/ http://www.sikulix.com/). This is also scriptable, e.g. in Python, but actually quite painful in my experience: When you GUI needs significant time for loading and rendering, the Sikuli program might fail to find a button that did not appear yet. So you need to play with timeouts and trial-and-error.