Content-Negotiation

HTTP headers that describe which formats a client is willing to receive.

Accept headers let the server negotiate a reply

Negotiation selects among representations the server can actually produce. Request preferences are inputs to that decision, not evidence that the response will satisfy them. A client still needs to inspect the selected representation. Vary records the request dimensions relevant to reuse; it does not identify the chosen format, language, or compression method. ...

A protocol that returns distinct representations from one URL based on request headers.

Accept plus Vary let one URL answer in several formats

One resource can have an HTML view for people and a JSON representation for programs. Sharing a URL is useful only when the origin and its intermediaries preserve that distinction. The endpoint must select an available format, serialize it honestly, describe its cache variation, and validate the selected representation rather than some other version of the same resource. ...