In a cross-domain context, when you are expecting the browser to make its XHR for JSON data via JavaScript to a third-party API. Think about making a call to th

In a cross-domain context, when you are expecting the browser to make its XHR for JSON data via JavaScript to a third-party API. Think about making a call to the Flickr.com HTTP API from your own WordPress myblog.com domain. In that context, the XHRs didn't support POST/PUT. Remember, even the fetch() API is pretty recent. POST-based forms are irrelevant for API usage. This MDN doc covers some of the complexity: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS And here is when browsers added support for CORS: https://caniuse.com/#feat=cors https://caniuse.com/#feat=cors