Cross-Origin Resource Sharing (CORS) is an HTTP-header based mechanism that 
allows a server to indicate any other origins (domain, scheme, or port) than 
its own from which a browser should permit loading of resources. CORS also 
relies on a mechanism by which browsers make a “preflight” request to the 
server hosting the cross-origin resource, in order to check that the server 
will permit the actual request. In that preflight, the browser sends headers 
that indicate the HTTP method and headers that will be used in the actual 
request.