Service Worker test
First image: On the first request, the service worker isn't registered when loading the image - so you will see a cat. If you access page a second time, you will see a unicorn (same image URL is referred in source code).
Also, if you access the image directly, you will see a unicorn, because the service worker is already active (try this with a fresh incognito window, and you will see the cat).
Service worker is installed:
Service worker is ready:
Unsing https://modernizr.com/ to check if Browser supports Service Workers
(if text is green, browser supports service workers)