If the server sends a response with an Access-Control-Allow-Origin value that is an explicit origin (rather than the "*" wildcard), then the response should also include a Vary response header with the value Origin — to indicate to browsers that server responses can differ based on the value of the Origin request header.
From https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin#CORS_and_caching:
Is this necessary?