There are some requests that are handled in a “simple” way. All GET requests belong to this group.

Also some POST and HEAD requests do as well.

POST requests are also in this group, if they satisfy the requirement of using a Content-Type of

application/x-www-form-urlencoded
multipart/form-data
text/plain
All other requests must run through a pre-approval phase, called preflight. The browser does this to determine if it has the permission to perform an action, by issuing an OPTIONS request.