alt Hacker NewsI've been wanting this from Cloudflare for years.
The classic problem here is if you do that thing where user agents that send "accept: text/html" get HTML, while user agents that don't get JSON or some other format.
This used to be impossible to deploy behind Cloudflare caching, because they ignored the Vary header on anything other than images - so you risked caching the JSON version and then serving it up to someone who was expecting HTML.
(Independent of the Cloudflare feature I ended up deciding never to use that pattern, because I prefer having URL that predictably returns HTML or JSON - I add a .json suffix to my apps to serve JSON instead.)
Frankly, the supposed variability of the Accept header never really sat all that well with me; in practice I much prefer working with explicitly versioned endpoints — one of the most infuriating things is having to hardcode "Accept: text/x-myorgname-custom-json-blob-v4" because omitting it would produce "406 Not Acceptable". Bonus points if that's the only Accept header the service would ever accept in all of three years of it working before being decommissioned. Double bonus points if v5 would be introduced behind a separate URI anyway (and it, too, would require precisely "Accept: text/x-myorgname-custom-json-blob-v5" and nothing else).