It's under the control of the origin site (for client manipulation there are stable extension apis in most browsers for this already). The primary use case is

It's under the control of the origin site (for client manipulation there are stable extension apis in most browsers for this already). The primary use case is caching and offline / partial connectivity, but it's full javascript and the service worker can construct arbitrary responses. Here's a good overview: https://www.youtube.com/watch?v=4uQMl7mFB6g https://www.youtube.com/watch?v=4uQMl7mFB6g