Content negotiation itself is not a trick, but this usage of it is fair to characterize as a sort of trick. Most uses of content negotiation are about serving the same content in different formats, like different image formats; in this case though, it's actually negotiating a different thing entirely, where one is a machine-readable CSV and the other a human-readable hypertext document. The fact that it works is unexpected to the uninitialized, because even just reading the RFC will not necessarily make it apparent why cURL and the browser would return such different results. There's a lot of ways a server could detect cURL, like the user agent, and it might come as a surprise to some that cURL and other user agents that are not web browsers often send */* for the accept header. It is not a terribly new way to use content negotiation, but arguably still a clever one. It's a deliberate behavior.