https://cosarara.me/misc/jxljs/ in my test, it will simply fall back to jpeg, even if the jxl.js library is loaded (scroll down to Image with fallback).
This is because JXL.js doesn't support `<picture>` tags - there is no rationale for WASM-decoding JXL when you can provide a fallback in natively-decoded JPEG/PNG/WebP.
The rationale would be that it might be more efficient to WASM-decode JXL than to download a JPEG, especially in pages with lots of images, but I would want a fallback if the browser does not support WASM.
