Why doesn't adding CORS headers to an OPTIONS route allow browsers to access my API?

Why doesn't adding CORS headers to an OPTIONS route allow browsers to access my API?

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express

Request header field Access-Control-Allow-Headers is not allowed by itself in preflight response

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express

No 'Access-Control-Allow-Origin' - Node / Apache Port Issue

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express

How to enable cross-origin resource sharing (CORS) in the express.js framework on node.js

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express

Allow CORS REST request to a Express/Node.js application on Heroku

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express

CORS pre-flight comes back with Access-Control-Allow-Origin:*, browser still fails request

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express

CORS Error: “requests are only supported for protocol schemes: http…” etc

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express

How to enable cors nodejs with express?

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express

Allow multiple CORS domain in express js

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express

AngularJS $resource makes HTTP OPTIONS request instead of HTTP POST for $save method

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express

Getting express server to accept CORS request

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express

XmlHttpRequest getAllResponseHeaders() not returning all the headers

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express

Enabling CORS in Create React App utility

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express

Allow CORS for PUT in Node.js

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express

Difference between 'Allow' and 'Access-Control-Allow-Methods' in HTTP response header?

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express