The CORS spec requires preflight requests that use the HTTP method OPTIONS.
This page, and the JS generated by it, attempts to use the HTTP method OPTION. But there is no such HTTP method and this is not a correct CORS preflight request.
Can you provide a link to test-cors.org that demonstrates this issue? You can make HTTP OPTIONS requests when doing CORS; a preflight is identified when the HTTP method is OPTIONS and the Access-Control-Request-Method header are present.
When I go to http://client.cors-api.appspot.com/client I see OPTION not OPTIONS, and when I look at the JS code it shows in the code pane I see a line that says "var method = 'OPTION';" not "var method = 'OPTIONS';"
It seems like the web app is missing the "S" everywhere, which will cause it to generate preflight requests that are not recognized as preflight requests.
For instance, try setting the header to "Access-Control-Request-Method:GET" and the URL to "https://cors-test.appspot.com/test". This will work with OPTIONS, but not with OPTION.
This is a very handy web app, by the way. Sorry to be a bit terse.
Fixing typo of OPTIONS. fixes #4
Thanks for the report! This is fixed on the live site now.