Jake Archibald takes a deep dive into CORS:
CORS is hard. It’s hard because it’s part of how browsers fetch stuff, and that’s a set of behaviours that started with the very first web browser over thirty years ago. Since then, it’s been a constant source of development; adding features, improving defaults, and papering over past mistakes without breaking too much of the web.
To test things out, Jake created “The CORS Playground”.