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).
Second image: This is inserted after the service worker is ready and with a delay of one second. Even if the cat file is referenced in the HTML, you will see a unicorn here on your first request.

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 to check if Browser supports Service Workers
(if text is green, browser supports service workers)