CORS (Cross-Origin Resource Sharing) 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.
Anyway, I figured I'd write down pretty much everything I know about CORS, and to make things interactive, I built an exciting new app.
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