Browsers tend to advertise what formats they accept outside of a very basic set to the server on requests via the HTTP Accept header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Ac...

For example, Firefox on macOS sent this Accept header for this request: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,/;q=0.8

Chrome sent text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,/;q=0.8,application/signed-exchange;v=b3;q=0.9