> The difference between HTTP/2 and HTTP/1.1 is exactly like the difference between plugging your PC in with a green cable or a red cable. The client neither knows nor cares.

Just like protocol negotiation, HTTP has format negotiation and XML postprocessing for exactly the same reason.

> So what do I put as the URL of an MP3 and an Ogg of the same song? It's the same content, just supplied in a different format

Whatever you want? If I access example.org/example.png, most websites will return a webp or avif instead if my browser supports it.

Similarly, it makes sense to return an XML with XSLT for most browsers and a degraded experience with just a simple text file for legacy browsers such as NCSA Mosaic or 2027's Google Chrome.