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.