A CORS preflight request is an HTTP OPTIONS request and a part of the CORS system that checks to see if the CORS protocol is understood and a server is aware using specific methods and headers. Read more on MDN documentation page.
Preflight requests are only issued on so-called 'complex' requests. An HTTP request is a complex request if one of the following requirements are met:
- A request that uses methods other than GET, POST, or HEAD
- A request that includes headers other than Accept, Accept-Language, or Content-Language.
- A request that has a Content-Type other than application/x-www-form-urlencoded, multipart/form-data, or text/plain
Generally, you want to avoid making preflight requests to the CDN as it adds another network round trip before the actual heavy lifting occurs.
Tinify CDN intelligently detects preflight requests and ensures they are properly forwarded to the origin server; this is done to avoid blocking any functionality.