I tried jxl.js, it was very finicky on iPad, out of memory errors [0] and blurry images [1]. In the end I switched to a proxy server, that reencoded jxl images into png.
Both issues seem to have known workarounds that could have been integrated to support JXL on iOS properly earlier than by waiting on Apple (who integrated JXL in Safari 17 apparently), so if anything that's a success story for "provide polyfills to support features without relying on the browser vendor."