[dspillett]

It is possible to polyfill an image format, this was done with FLIF¹². Not that it mean FLIF got the traction required to be used much anywhere outside its own demos…

It is also possible to detect support and provide different formats (so those supporting a new format get the benefit of small data transfer or other features) though this doesn't happen as it isn't usually an issue enough to warrant the extra complication.

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[1] Main info: https://flif.info/

[2] Demo with polyfill: https://uprootlabs.github.io/poly-flif/

[afavour]

Any polyfill requires JavaScript which is a dealbreaker for something as critical as image display, IMO.

Would be interesting if you could provide a decoder for <picture> tags to change the formats it supports but I don't see how you could do that without the browser first downloading the PNG/JPEG version first, thus negating any bandwidth benefits.

[dspillett]

Depending on the site it might be practical to detect JS on first request and set a cookie to indicate that the new format (and polyfill) can be sent on subsequent requests instead of the more common format.

Or for a compiled-to-static site just use <NOSCRIPT> to let those with no JS enabled to go off to the version compiled without support/need for such things.