[Joker_vD]

That's tangentially related to the Vary header. Not only Accept can go into its value, you know.

And an optimal solution IMHO would be for the origin server to simply return 302 to a specific resource, selected upon the value of the Accept header:

    GET /thumb.php?id=kekw HTTP/1.1
    Accept: image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8

    HTTP/1.1 302 Found
    Location: /media/thumb.jpg?id=kekw
    Vary: Accept

    GET /media/thumb.jpg HTTP/1.1
    Content-Type: image/jpeg

[bmandale]

Sure, except I doubt most people want to uglify all their urls with extensions for occasional alternates. Plus, if the url with the extension gets past around instead of the original (as would inevitably be done) you're back to square one.

I had thought about recommending that people just use an alternate link as intended, to point to an alternate format. I think that would work best using existing web standards as intended, but it has the downside of initially serving the original format regardless of the content type.

[Joker_vD]

> if the url with the extension gets past around instead of the original (as would inevitably be done) you're back to square one.

Why? It has no "Vary" header, and it's the one that's supposed to get cached anyhow.

[bmandale]

If people see it in the url bar and copy paste it from there. In the case of images if they "copy image url".