“Proxy servers such as NGinx and Varnish: . . . if the Accept header is omitted, the proxy cache can return any of the cached responses.”

This is not really my area of expertise, but this behaviour is inconsistent with my understanding of how Accept and Vary are supposed to work in general. I would expect a cache to treat a missing Accept field as a separate “value” that does not match any specific Accept value.

See <https://www.w3.org/mid/20040223204041.GA32719@mail.shareable.org>. Also, what about a server that sets “Vary: Cookie”, to send a response that depends on whether the user has already seen the page. Do these NGinx and Varnish caches respond with a random response if Cookie is missing?

I still think if you care about the media type, it is better practice to specify what types you want with a more explicit Accept value. And if you don’t care about the media type, the NGinx/Varnish behaviour may not be a problem anyway.