[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.