A *PDF* with embedded JPEG 2000 data should, as far as I know, decode in modern browser PDF viewers. PDF.js and PDFium both are using OpenJPEG. But despite that, browsers don't currently support JPEG 2000 in general.

I'm saying this to explain how JPEG XL support in PDF isn't a silver bullet. Browsers already support image formats in PDF that are not supported outside of PDF.